Gift   of 
Pi1.    Monica  Briner 


A  TEXTBOOK  OF 

GENERAL  EMBRYOLOGY 


BY 
WILLIAM  E.  KELLICOTT 

PROFESSOR    OF   BIOLOGY   IN    GOUCHER    COLLEG] 


NEW  YORK 
HENRY  HOLT  AND  COMPANY 

1      ;  -  '   1'9V3  "      -  — 


COPYRIGHT.   1913, 

BY 
HENRY  HOLT  AND    COMPANY 


*.A 


PREFACE 

I 

General  embryology  should  occupy  an  important  place  in  the 
collegiate  study  of  biology.  In  no  other  connection  are  the 
essential  phenomena  of  life  better  illustrated,  in  no  other  form 
are  they  more  readily  appreciated.  The  facts  of  embryology 
lead  directly  to  the  great  problems  of  the  science  of  biology  as 
it  exists  to-day,  and  many  fundamental  biological  conceptions 
either  are  directly  connected  with,  or  are  illuminated  by,  the 
study  of  the  early  phenomena  of  individual  development. 

The  author's  experience  has  clearly  indicated  that  the  subject 
has  this  value  as  a  collegiate  study.  Indeed,  the  book  is  the 
direct  outgrowth  of  such  experience,  and  it  has,  in  substance, 
been  in  use  as  such  a  text  for  several  years.  In  its  present  form 
it  is  hoped  that  it  will  be  found  useful  to  the  student  who  is 
endeavoring  to  comprehend  the  general  principles  of  the  science 
of  life,  as  well  as  to  the  student  preparing  for  the  professional 
study  of  some  field  of  biology  or  of  medicine. 

Its  design  as  a  textbook,  rather  than  as  a  handbook,  accounts 
for  certain  characteristics.  The  topics  considered  have  through- 
out been  approached  from  the  standpoint  of  their  general 
biological  relations,  and  in  the  selection  of  the  facts  mentioned 
and  the  topics  discussed,  as  well  as  in  the  style  and  method  of 
presentation,  the  student  has  been  first  in  mind.  The  arrange- 
ment of  the  subject  matter  in  two  sizes  of  type  may  prove 
useful  for  those  undertaking  a  brief  course.  In  a  few  instances 
this  has  involved  slight  repetition,  but  repetition  is  not  always 
a  pedagogic  evil. 

At  the  end  of  each  chapter  will  be  found  a  list  of  references  to 
literature.  Usefulness  to  the  student  has  been  the  only 
criterion  in  determining  the  admission  of  titles  to  these  lists. 
Consequently  there  will  be  found  titles  of  works  of  historical 
importance,  of  recent  works  containing  contributions  of  impor- 

iii 


1 8}  4 


iv  PREFACE 

tance  or  representing  present  tendencies  in  research,  and  of 
papers  containing  extensive  literature  references,  valuable 
illustrations,  or  general  summaries.  As  far  as  possible  the  lists 
contain  references  to  works  presenting  both,  or  several,  sides 
of  mooted  questions  mentioned  in  the  text.  There  will  also  be 
found,  in  nearly  every  instance,  the  titles  of  papers  from  which 
illustrations  may  have  been  taken. 

To  a  large  extent  the  figures  have  been  redrawn,  from  the 
original  sources,  for  this  work:  it  is  a  pleasure  to  notice  the 
uniform  courtesy  with  which  authors  have  granted  permission  to 
make  this  use  of  their  illustrations.  The  following  special  debts 
are  gratefully  acknowledged:  to  Prof.  Edmund  B.  Wilson  and 
The  Macmillan  Company,  for  cliches  and  for  permission  to 
copy  a  considerable  number  of  illustrations  in  their  "The  Cell 
in  Development  and  Inheritance";  to  Prof.  Gary  N.  Calkins, 
The  Macmillan  Company,  and  Lea  and  Febiger,  for  cliches  and 
for  permission  to  copy  certain  illustrations  in  their  "The 
Protozoa"  and  "Protozoology";  to  Prof.  Ulric  Dahlgren,  Prof. 
William  A.  Kepner,  and  The  Macmillan  Company,  for  permis- 
sion to  copy  certain  illustrations  in  their  "Principles  of  Animal 
Histology";  to  Prof.  J.  W.  Jenkinson  and  the  Delegates  and 
Secretary  of  the  Clarendon  Press,  for  cliches  from  their  "  Experi- 
mental Embryology";  and  finally  to  Herr  Gustav  Fischer  and 
to  the  several  authors,  for  cliches  and  for  permission  to  copy  or 
otherwise  make  use  of  illustrations  from  Korschelt  and  Heider's 
"Lehrbuch,"  Oscar  Hertwig's  "Handbuch,"  Doflein's  "Proto- 
zooenkunde,"  and  Ziegler's  "Lehrbuch."  In  every  instance 
specific  reference,  both  to  the  immediate  and  the  ultimate 
sources  of  the  figures  borrowed,  is  made  in  the  legends.  I 
desire  also  to  acknowledge  my  indebtedness  to  the  authorities 
of  The  Johns  Hopkins  University,  for  the  use  of  valuable 
library  facilities. 

W.  E.  K 

BALTIMORE,  MD., 
March,    1913. 


CONTENTS 

CHAPTER  I 

PAGE 

ONTOGENY     1 

CHAPTER  II 

THE  CELL  AND  CELL  DIVISION 31 

CHAPTER  III 

THE  GERM  CELLS  AND  THEIR  FORMATION 85 

CHAPTER  IV 
MATURATION 131 

CHAPTER  V 
FERTILIZATION 164 

CHAPTER  VI 

CLEAVAGE 219 

CHAPTER  VII 

THE   GERM  CELLS  AND  THE  PROCESSES  OF  DIFFERENTIATION,   HEREDITY, 

AND  SEX  DETERMINATION 260 

CHAPTER  VIII 

THE      BLASTULA,      GASTRULA,      AND      GERM      LAYERS.       MORPHOGENETIC 

PROCESSES 329 

INDEX  .   367 


TEXT-BOOK  OF 
GENERAL  EMBRYOLOGY 

CHAPTER  I 

ONTOGENY 

LIVING  organisms  come  into  existence  only  as  the  offspring 
of  preexisting  living  organisms  of  the  same  kind  or  species. 
Aristotle's  belief  that  eels  were  generated  from  mud  and  slime 
is  represented  to-day,  in  many  youthful  minds,  by  the  firm 
conviction  that  a  horse  hair,  if  only  kept  long  enough  in  water, 
will  surely  "turn  to  a  worm."  From  the  time  of  Redi  the 
belief  that  the  living  might  be  generated  from  the  wholly 
non-living  gradually  became  restricted,  in  its  application,  to 
lower  and  still  lower  groups  of  organisms.  For  a  long  time  it 
remained  applied  only  to  those  forms  at  the  lower  limit  of  the 
living— the  bacteria.  From  this  position  the  belief  that "  spon- 
taneous generation"  of  organisms  occurs  nowadays,  was 
finally  driven  by  the  brilliant  demonstrations  of  Pasteur  and 
Tyndall  that  even  these  simplest  and  smallest  of  organisms 
arise  only  from  preexisting  living  organisms  of  the  same  kind. 

This  property  of  producing  new,  specifically  similar  individ- 
uals is  one  of  the  few  really  distinctive  characteristics  of  living 
things,  and  since  the  newly  produced  resemble  closely  the 
parent  form,  we  speak  of  the  property  as  Reproduction.  The 
fact  that  at  corresponding  ages  offspring  resemble  their  parents, 
is  the  fact  of  heredity.  But  when  these  offspring  are  first 
distinguishable  as  separate  and  new  individuals  they  bear  little 
or  no  visible  resemblance  to  the  adult  organisms  producing 
them.  This  resemblance  appears  gradually,  as  the  result  of  a 
long  series  of  processes,  complex  and  often  very  special,  in- 
volving changes  in  structure,  function,  and  form,  only  at  the 
conclusion  of  which  has  the  new  organism  reproduced,  more 

1 


GENERAL  EMBRYOLOGY 


or  less  precisely,  the  form  and  other  characteristics  of  its 
parents.  The  facts  regarding  all  of  these  processes  of  develop- 
ment, of  the  external  and  internal  changes  in  form  and  structure 
of  the  new  organism,  of  the  complex  chain  of  processes  leading 
to  its  first  formation,  and  of  the  role  of  external  factors  through- 
out all  of  these,  constitute  the  science  of  Embryology.  We 
may  define  Embryology  briefly  then,  as  the  science  of  the 
genesis  of  the  adult  organism. 

As  a  general  introduction  to  this  subject  we  may  first  sketch 
in  outline  the  broad  features  of  reproduction  among  the  lower 
animals,  mentioning  but  a  few  of  the  almost  infinitely  varied 
forms  which  this  process  assumes.  It  should  be  understood  in 


<^ife  * 

3--VX2 


c 


FIG.  1. — Simple  fission  in  Amoeba  vespertilio.  After  Doflein.  A.  Normal 
vegetative  form.  B.  Commencement  of  fission  ("biscuit  form").  C.  Fission 
nearly  completed;  separation  of  daughter  cells,  z,  cells  of  an  Alga,  Zoochlorella. 

advance  that  the  series  of  reproductive  processes,  of  increasing 
complexity,  to  be  outlined,  has  little  if  any  phyletic  significance; 
this  arrangement  is  made  for  comparative  purposes  alone. 

The  simplest  and  apparently  the  most  primitive  mode  of  re- 
production is  that  known  &s  fission,  characteristic  of  the  single- 
celled  organisms,  the  Protozoa  and  Protophyta.  In  the  case  of 
simple  or  binary  fission  a  separation  of  the  nuclear  material  of 
the  cell  into  two  separate  masses  is  followed  by  a  constriction 


ONTOGENY  3 

of  the  cell  body  into  two  bodies,  each  of  which  may  then  form 
parts  corresponding  with  those  carried  away  by  its  sister 
cell,  and  finally  develop  into  a  creature  resembling  the  original 
organism  (Fig.  1).  In  many  unicellular  forms,  particularly 
among  the  Sporozoa,  a  process  of  multiple  fission  or  brood 
formation  occurs.  This  is  frequently  preceded  by  growth  of 
the  organism  to  an  unusual  size;  then  an  uninterrupted  series 
of  simple  fissions,  without  intermediate  growth  or  development 
on  the  part  of  the  daughter  cells,  results  in  the  formation  of  a 


m 


cy 


FIG.  2. — Multiple  fission  in  a  parasitic  Infusorian,  Holophrya  multifiliis. 
After  Hatschek.  A.  Normal  vegetative  individual.  B.  Cyst  containing  the 
products  of  repeated  binary  fission;  some  of  the  zoo  spores  are  shown  leaving  the 
cyst.  C.  One  of  the  zoospores,  enlarged,  cr,  contractile  vacuole;  cy,  cyst; 
m,  mouth;  ma,  macronucleus;  mi,  micronucleus ;  t,  zoospores. 

large  number  of  small  organisms.  These  usually  remain 
associated,  frequently  within  a  cyst  formed  by  the  parent  cell, 
until  the  whole  process  of  fission  is  completed  (Fig.  2).  In 
other  cases  the  nucleus,  or  nuclei,  alone  may  divide,  either 
successively  or  simultaneously,  into  a  large  number  of  separate 
nuclei;  the  cytoplasm  immediately  surrounding  each  nucleus  is 
then  cut  out  as  a  separate  cell,  so  that  the  organism  appears  to 
fragment  simultaneously  into  a  large  number  of  small  daughter 
organisms  (Fig.  3).  The  number  of  new  individuals  formed  in 
this  way  may  vary  from  four,  as  in  some  Infusoria,  up  to  as 
many  as  several  hundred  in  many  different  forms,  especially 
among  the  Sporozoa.  When  the  number  is  large  the  process 


4  GENERAL  EMBRYOLOGY 

is  more  frequently  termed  spore  formation  or  sporulation  and 
the  products  are  known  as  swarmspores  or  zoospores.  Tech- 
nically the  term  sporulation  or  sporogony  (metagametic  division) 
is  used  only  when  this  process  of  multiple  fission  occurs  sub- 
sequently to  a  process  of  cell  fusion.  If  no  such  process  of 
cell  fusion  or  conjugation  has  preceded,  the  process  is  termed 
schizogony  (agamogony) ,  and  the  products  of  the  division  are 
called  schizonts  (agamonts). 


FIG.  3. — Multiple  fission  (schizogony)  in  the  Sporozoan,  Coccidium  schubergi. 
After  Schaudinn.  A.  Entrance  of  organism  ("sporozoite")  into  epithelial  cell 
of  host.  B,  C.  Two  stages  in  growth.  D.  Multiple  division  of  nucleus.  E. 
Daughter  nuclei  superficial,  cytoplasm  still  undivided.  F.  Fission  of  the  super- 
ficial cytoplasm  surrounding  the  nuclei,  leaving  a  central  undivided  mass.  G. 
Free  schizonts  ("merozoites"). 


Reproduction  by  analogous  processes  also  called  fission, 
simple  or  multiple,  is  only  occasional  among  the  many-celled 
animals.  In  a  few  such  forms  the  entire  body  separates  as  a 
unit  into  two  or  more  organisms.  Organs  and  tissues  in  the 
plane  of  separation  are  divided,  and  each  of  the  daughter  or- 
ganisms then  regenerates  parts  corresponding  to  those  carried 


ONTOGENY 


off  by  the  other,  forming  new  individuals  smaller  than  the 
parent  form,  but  otherwise  similar  to  it  (Fig.  4).  This  fission 
in  the  Metazoa  is  not  really  comparable  with  the  similarly 
named  process  in  the  Protozoa;  it  represents  a  special  acquire- 
ment and  is  usually,  though  not  always,  associated  with  other 
more  complex  modes  of  reproduction.  Normal  fission  is 
known  to  occur  in  many  genera  among  the  Coelenterates,  and 
less  frequently  among  the  Porifera,  Platyhelminthes,  Annulata, 
Bryozoa,  and  Echinoderms. 

It  is  obvious  that  in  reproduction  by  fis- 
sion the  individuality  of  the  parent  organism 
is  lost  in  the  act  of  giving  rise  to  the  new 
individuals,  although  none  of  the  substance     ] 
of  the  original  organism  perishes  in  the  act. 


FIG.  4. — Fission  in  Metazoa.  A,  B.  Two  stages  in  the  transverse  fission  of 
the  Actinian,  Gonactinia  prolifera.  From  Korschelt  and  Heider,  after  Blochmann 
and  Hilger.  C.  Successive  transverse  fissions  in  the  Platyhelminth,  Micro- 
stomum  lineare.  After  L.  von  Graff.  /,  //,  III,  mark  the  levels  of  the  successive 
fissions;  a  fourth  fission  also  is  indicated,  p,  p,  pharynges. 

In  the  fission  of  Metazoa  usually  many  of  the  structures  of  the 
parent  are  simply  transferred  to  the  new  organisms  with  com- 
paratively little  differentiation  of  parts  anew,  out  of  a  visibly 
undifferentiated  condition.  In  the  Protozoa  this  may  or 
may  not  be  the  case.  In  some  of  the  highly  organized 
Ciliata  most  of  the  structural  differentiation  disappears  just 
previous  to  fission,  after  which  each  daughter  cell  differ- 
entiates a  typical  form  and  structure  anew  (Fig.  5).  In 
other  Protozoa  there  is  a  considerable  transference  of  char- 
acteristics accompanied  by  a  lesser  amount  of  regeneration  or 


6 


GENERAL  EMBRYOLOGY 


redifferentiation.  After  multiple  fission  or  speculation,  the 
end  products  of  the  process  are  commonly  minute  and  visibly 
quite  unlike  the  adult  form;  this  they  come  to  resemble  only 
through  growth  and  differentiation,  that  is,  through  processes 
of  true  development. 

A  reproductive  process  closely 
allied  to  fission  is  the  familiar  proc- 
ess of  budding.  Here  one  or 
several  small  outgrowths  or  ''buds" 
are  produced  from  some  portion  of 
the  parent  organism,  and  develop 
into  forms  resembling  the  parent, 
either  before  or  after  becoming  de- 
tached. This  process  frequently  ap- 
pears as  a  sort  of  unequal  fission 
and  may  indeed  rightly  be  regarded 
as  such;  but  usually  so  great  is  the 
disparity  in  size,  as  well  as  in  extent 
of  differentiation,  between  parent 
and  buds,  that  the  processes  are  prop- 
erly distinguished.  Budding  occurs 
in  many  Protozoa  (e.g.,  Ephelota 
(Fig.  6,  A),  some  Rhizopoda),  and 
is  quite  frequent  among  the  lower 
Metazoa,  particularly  in  colony  form- 
FIG.  5.— Binary  fission,  in  ing  species.  It  occurs  among  the 

the  Infusorian,  Euplotes  harpa.     pnrifprfl     Prplpntprflt*      PlfltvhplTYiiTV 
After     Wallengren.       Division    rc       era>    ^Q  eraia,    natyn< 

stage  immediately  before  the    thes,  Annulata,  Bryozoa,  and  Tuni- 

separation  of  the  daughter  cells.  „..         .      _x         T       ,       ,  ,. 

cata  (Fig.  6,  B).     In  budding  there 

is  ordinarily  very  little  direct  transference  of  structures, 
the  development  of  the  bud  occurring  after  it  has  been 
completely  delimited  as  a  comparatively  undifferentiated  mass; 
its  development  may  then  be  almost  complete  before  the 
separation  of  the  two  organisms.  Nor  does  this  process  involve 
necessarily  the  loss  of  identity  of  the  parent,  which  may  con- 
tinue to  live  and  produce  buds  for  a  considerable  period. 
In  a  few  Protozoa,  fresh  water  sponges,  Trematodes  and 


ONTOGENY  7 

Bryozoa,  internal  buds  are  formed  within  the  parent  cell  or 
body  (Fig.  7).  These  become  free  and  develop  into  new  indi- 
viduals ordinarily  only  after  the  death  of  the  parent  body. 
Although  the  formation  of  these  gemmules  (Porifera),  or  stato- 
blasts  (Bryozoa)  suggests  that  form  of  reproduction  character- 
istic of  the  higher  Metazoa,  i.e.,  through  internally  forme'd  germ 
cells,  it  will  appear  later  that  the  two  processes  are  not  at 
all  to  be  compared. 


FIG.  6. — Reproduction  by  budding.  A.  In  the  Infusorian,  Ephelota.  From 
Calkins,  "Protozoa."  N,  branched  macronucleus  extending  from  the  parent 
cell  into  each  bud.  B.  In  the  Tunicate,  Doliolum.  After  Neumann.  Ventral 
outgrowth  of  a  phorozooid.  6,  reproductive  buds  of  various  ages;  g,  germinal 
knob;  s,  stalk. 

In  some  of  the  shelled  Rhizopods  (Euglypha,  Arcella)  a 
combination  of  the  processes  of  budding  and  fission  occurs. 
In  this  process,  called  bud-fission,  about  one-half  of  the  proto- 
plasm flows  outside  the  original  shell,  which  these  forms  possess, 
and  secretes  a  new  shell  like  that  of  the  parent  form;  or  the 
rudiments  of  the  new  shell  may  be  formed  before  the  appear- 
ance of  the  bud.  The  cell  body  then  divides  and  the  two  similar 
individuals  may  either  separate  completely,  or  they  may 
remain  in  contact,  forming  after  repeated  bud-fission,  an 
aggregate  of  related  though  distinct  organisms. 

In  many  of  the  Protozoa  multiple  fission  may  be  incomplete, 


8 


GENERAL  EMBRYOLOGY 


or  the  daughter  cells,  not  scattering  as  separate  individuals, 
may  remain  associated  during  division  and  growth,  forming  a 
colony  of  organically  related  cells  which  as  a  whole  is  to  be 
regarded  as  the  individual  organism.  Among  nearly  all  of 
these  colonial  or  compound  forms  the  entire  colony,  or  cwno- 


FIG.  7. — The  formation  and  development  of  the  statoblast,  in  the  fresh  water 
Bryozoan,  Cristatella.  A.  After  Braem.  Others,  after  Verworn.  A.  Longi- 
tudinal section  through  the  funiculus,  showing  the  relations  of  the  statoblast. 
B-E.  Optical  sections  of  stages  in  the  development  of  the  statoblast.  F.  Sec- 
tion showing  rudiment  of  embryo,  e,  superficial  ectoderm;  i,  inner  layer  of 
funiculus  (ectoderm) ;  o,  outer  layer  of  funiculus  (endoderm) ;  s,  rudiment  of 
statoblast. 

bium  as  it  is  called,  is  not  later  involved  as  a  unit  in  reproduc- 
tion, but  the  component  cells  may  individually  undergo 
fission  leading  to  the  formation  of  new  colonies.  In  forms  like 
Pandorina  or  Platydorina  all  the  cells  are  alike,  and  each  repro- 
duces, so  that  there  may  be  as  many  new  colonies  formed  as 
there  are  cells  in  the  original  colony  (Fig.  8).  In  other  forms 
the  power  of  reproduction  is  limited  to  certain  cells,  termed 


ONTOGENY  9 

gonidia,  and  there  results  a  sharp  distinction  between  repro- 
ductive and  vegetative  cells  (Fig.  9).  Thus  in  Pleodorina,  of 
the  thirty-two  cells  forming  the  colony,  four  anterior  cells  are 
purely  vegetative,  the  remaining  twenty-eight  are  gonidial; 


FIG.  8. — Reproduction  in  the  colonial  Flagellate,  Pandorina  morum.  From 
Hertwig,  after  Pringsheim.  /.  Free-swimming  vegetative  colony.  II.  Daugh- 
ter colonies  formed  by  the  fissions  of  each  individual  in  a  colony  similar  to  the 
preceding.  ///.  Gamete  formation  in  a  colony  formed  as  above.  IV,  V,  VI. 
Stages  in  the  conjugation  of  gametes  to  form  a  zygote  (VI).  VII.  Zygote  after 
growth  to  full  size.  VIII,  IX.  Production  of  swarm  spores  by  the  zygote. 
X.  Vegetative  colony  formed  by  fissions  of  the  swarm  spore. 

while  in  Volvox,  where  the  fully  developed  colony  may  include 
as  many  as  twelve  thousand  or  more  individuals,  only  five  to 
fifty  cells,  scattered  through  the  posterior  half  of  the  colony  are 
gonidial,  the  remainder  being  vegetative  (Fig.  10). 


10  GENERAL  EMBRYOLOGY 

Among  most  of  these  colonial  Flagellates,  and  indeed  in 
many  other  Protozoan  groups,  at  irregular  intervals  these 
gonidial  cells  lose  the  property  of  directly  giving  rise  to  new 
colonies,  and  become  very  highly  differentiated  in  structure 
and  in  behavior.  These  highly  modified  gonidia  are  termed 
gametogonidia,  the  ordinary  gonidia  being  then  given  the  name 


FIG.  9. — Colony  of  the  Flagellate,  Pleodorina  illinoisensis.  Lateral  view. 
After  Kofoid.  A,  anterior;  g,  gonidial  (reproductive)  cells,  v,  vegetative 
cells. 

of  parthenogonidia.  The  gametogonidia  form  specialized  cells 
termed  gametes  which  must  meet  and  fuse  in  pairs,  i.e.,  con- 
jugate, before  reproduction  may  proceed.  In  the  simplest  cases 
these  two  gametes  are  nearly  or  quite  alike.  In  most  cases, 
however,  gametes  of  two  very  unlike  forms  are  produced  by 
different  gametogonidia.  These  must  conjugate  in  pairs  of 
unlikes  before  reproduction  is  possible.  A  series  of  forms 
illustrating  the  progressive  differentiation  of  the  gametes,  or 
germ  cells,  is  described  in  Chapter  V.  For  the  present  we  may 
merely  notice  that  in  such  a  colonial  form  as  Volvox,  under 
certain  conditions,  the  parthenogonidia  cease  reproducing; 
certain  of  them  (odgonidia  or  ovaries)  enlarge,  and  each  differ- 
entiates a  large  non-motile  cell,  the  ovum,  or  odsphere,  or 
macrogamete,  while  others  (spermagonidia,  or  spermaries)  after 
enlarging  similarly,  divide  repeatedly,  each  forming  a  large 
number,  often  as  many  as  one  hundred  and  twenty-eight, 


ONTOGENY 


11 


small  actively  motile  cells  termed  sperm  cells  or  microgametes. 
A  single  sperm  from  one  colony  then  meets  and  conjugates 
with  a  single  ovum  from  another  colony,  forming  thus  a  single 


D 


FIG.  10. — Reproduction  in  the  colonial  Flagellate,  Volvox.  A.  After  Prings- 
heim,  B-E,  after  Klein  and  Schenck.  A.  "Young  colony  showing  distinction 
between,  s,  somatic  cells,  and  g,  reproductive  cells  (parthenogonidia).  B.  Older 
colony  showing,  p,  parthenogonidia,  o,  oogonidia,  sp,  spermagonidia  in  various 
stages  of  formation.  C.  Spermagonidium  consisting  of  thirty-two  spermagam- 
etes,  seen  from  the  side  in  D.  E.  Spermagametes  more  highly  magnified. 

cell  or  zygote,  which  through  continued  fission  gives  rise  to  the 
individuals  of  a  new  colony.1 

1  Useful  diagrams  of  these  forms  of  reproduction  and  gamete  formation 
are  given  in  HEGXER,  "An  Introduction  to  Zoology,"  New  York,  1910,  pp. 
112-115. 


12  GENERAL  EMBRYOLOGY 

Reproduction  following  gamete  formation  and  fusion  (syn- 
gamy)  is  commonly  known  as  "sexual"  reproduction,  while  the 
term  "asexual"  is  applied  to  modes  of  reproduction  which  do 
not  involve  the  fusion  of  gametes.  It  is  now  clear,  however, 
that  several  different  and  unrelated  processes  are  included  under 
each  of  these  heads.  Thus  as  "  asexual"  must  be  included 
such  diverse  modes  of  propagation  as  budding,  fission,  brood 
formation,  parthenogenesis,  development  from  spores,  and  so 
forth;  and  "sexual"  reproduction  would  also  follow  many 
forms  of  syngamic  fusion.  Two  further  conditions  tend  to 
rob  these  terms  of  their  precise  significance;  these  are,  the 
existence  of  transitional  conditions,  some  of  which  have  been 
mentioned  above,  and  the  doubtfully  essential  character  of 
the  primary  relation  of  syngamy  to  reproduction.  These 
useful  terms  are  to  be  retained  only  as  convenient  though 
inexact  expressions,  and  are  to  be  used  in  much  the  same  way 
that  we  still  employ  the  convenient  words  "vertebrate"  and 
"invertebrate."  Even  so,  we  may  avoid  any  unintentional 
implications  by  substituting  the  more  exact  terms  amphigony 
and  monogony  for  sexual  and  asexual  respectively. 

Without  suggesting  the  idea  of  direct  relationship  among  any 
existing  forms,  we  may  say  that  it  is  but  a  short  step  from  the 
processes  of  gamete  formation  and  reproduction  among  the 
colonial  Protozoa,  to  the  mode  of  reproduction  characteristic 
of  the  Metazoa,  which  after  all  may  be  considered  highly 
organized  cell  colonies.  One  of  the  more  distinctive  as  well  as 
more  obvious  characteristics  of  the  Metazoa  is  the  structural 
and  functional  differentiation  of  large  groups  of  cells  as  tissues, 
each  variety  of  tissue  performing,  in  the  animal  economy, 
chiefly  one  function,  such  as  conduction,  support,  or  excretion. 
Among  these  various  tissues  is  the  reproductive  or  germinal 
tissue,  which,  in  all  save  a  few  of  the  lower  Metazoa,  is  always 
in  the  form  of  definite  organs,  the  gonads.  The  distinction, 
suggested  by  some  of  the  colonial  Protozoa,  between  the  repro- 
ductive and  the  vegetative  tissues  was  probably  the  earliest  of 
those  "physiological  divisions  of  labor"  which  involved  tissue 
differentiations  in  the  Metazoa.  The  essential  cells  of  the 


ONTOGENY  13 

gonads,  which  correspond  functionally  to  garnet ogonidia,  divide 
repeatedly,  the  products  of  their  multiplication  being,  with  few 
exceptions,  ultimately  thrown  outside  the  body  as  the  highly 
modified  germ  cells  or  gametes.  In  all  the  Metazoa  these  germ 
cells  are  of  two  quite  unlike  forms;  the  ova  or  "eggs,"  are  large 
and  ordinarily  non-motile,  while  the  spermatozoa  or  sperm  cells , 
are  small  and  actively  motile.  These  Metazoan  germ  cells 
clearly  correspond  with  the  ova  or  macrogametes  and  the  sperms 
or  microgametes  of  certain  of  the  Protozoa.  The  gonads 
forming  the  ova  and  spermatozoa  are  known  as  the  ovaries 
and  testes  respectively.  In  most  of  the  Metazoa  germ  cells  of 
only  a  single  kind  are  formed  by  a  single  organism,  a  condition 
which  leads  to  the  primary  distinction  of  sex;  individuals 
forming  ova  are  called  females,  those  forming  spermatozoa, 
males. 

In  comparatively  few  kinds  of  animals  does  a  single  individual 
normally  possess  gonads  of  both  types,  and  thus  become  capable 
of  forming  both  kinds  of  germ  cells,  either  simultaneously  or 
successively.  Such  a  condition  is  known  as  hermaphroditism; 
it  occurs  chiefly  among  the  lower  Metazoa,  such  as  the  Platy- 
helminthes,  Nemertea,  some  Annulates  and  Tunicates,  and 
less  frequently  among  the  Molluscs,  Echinoderms,  Bryozoa, 
Brachiopoda,  and  Crustacea.  Among  the  Chordata  normal 
hermaphroditism  is  found  only  rarely  (some  Teleosts) ,  though  it 
may  occur  as  an  abnormality  in  any  group.  In  a  few  animals 
a  special  form  of  hermaphroditism  occurs,  where  a  single  gonad 
may  produce  first  sperm  and  later  ova,  a  condition  known  as 
protandry  and  found  in  some  Nematode  worms  and  in  the 
Cyclostome,  Myxine,  for  example.  The  process  of  forming 
first  ova  and  later  spermatozoa,  known  as  protogony,  is  very 
rare  among  animals. 

In  the  reproduction  of  all  except  a  very  few  of  the  Metazoa, 
the  initial  phase  is  the  union  of  an  egg  cell  and  a  sperm  cell; 
this  process  is  known  as  fertilization  or  syngamy,  and  the  double 
cell  thus  formed,  which  is  called  the  zygote  or  odsperm,  then 
gives  rise  directly  to  the  new  individual.  Not  all  of  the  germ 
cells  formed  by  an  organism  actually  happen  to  give  origin  to 


14  GENERAL  EMBRYOLOGY 

new  organisms,  although  any  may  do  so.  But  with  infrequent 
exceptions,  where  unusual  methods  of  reproduction  occur  at 
times,  a  few  of  which  have  been  noted  above,  new  Metazoan 
individuals  arise  only  from  the  union  of  two  germ  cells. 

The  substance  which  forms  the  reproductive  cells  or  gametes  of 
an  organism  is  called  the  germinal  substance,  or  briefly,  the  germ. 
This  is  visibly  distinguishable  at  a  very 'early  period  in  the 
existence  of  the  new  organism,  from  that  material  which  is  to 
form  all  of  the  remainder  of  the  organism,  in  distinction  known 
as  the  somatic  tissue  or  soma,  or  simply  as  the  body.  Among 
the  higher  forms  these  two  kinds  of  substance — germ  and  soma 
—have  very  different  histories  and  fates.  According  to  the 
theory  of  Germinal  Continuity,  elaborated  by  Weismann,  the 
germ  represents  or  contains  an  organic  substance  which  has 
been  in  a  living  state  since  the  beginning  of  life,  and  which 
must  continue  in  this  state,  in  some  form,  as  long  as  living 
things  shall  be  produced.  The  soma,  on  the  contrary,  is 
thought  to  be  built  up  around  the  germ,  anew  and  under  its 
influence,  in  each  generation  of  organisms.  Upon  the  death  of 
the  individual  it  is  destroyed  completely  as  living  substance; 
somatic  cells  finally  leave  no  descendants.  Thus  in  species 
which  reproduce  by  this  method  the  soma  or  body  is  wholly 
temporary,  while  the  germ  may  properly  be  said  to  be  poten- 
tially ever  enduring.  For  while  actually  the  greater  part  of  the 
germ  substance  formed  in  an  organism  is  destined  to  perish, 
either  before  the  body  or  with  it,  or  at  any  rate  with  the  race, 
some  germ  must  always  remain,  producing  the  generations  of 
the  future.  The  essentials  of  this  idea  are  expressed  in  the 
accompanying  diagram  (Fig.  11). 

On  account  of  its  usefulness  the  value  and  significance  of  the 
distinction  between  germ  and  soma  are  frequently  over- 
emphasized. In  many  organisms  the  distinction  can  scarcely 
be  drawn  at  all,  for  under  certain  conditions,  either  normal 
or  unusual,  cells  which  are  evidently  "  somatic "  may  take 
on  reproductive  characteristics  and  function  as  germ  cells. 
Many  such  cases  are  known  among  animals,  and  among  the 
plants  reproduction  from  somatic  tissues  and  cells  is  very 


ONTOGENY 


15 


common,  indeed  in  some  it  appears  to  be  the  normal  method 
of  reproduction. 

Among  the  Protozoa  this  familiar  distinction  between  germ 
and  soma  cannot  be  drawn  at  all.     In  these  simple  forms  the 


o  o  o  o 


FIG.  11. — Diagram  illustrating  the  theory  of  Germinal  Continuity.  A,  B,  C, 
represent  successive  generations;  g,  gamete  (sperm  cell)  produced  by  another 
organism;  z,  zygote.  White  circles  indicate  successive  cell  divisions  within  the 
somatic  tissues,  the  existence  of  which  terminates  with  the  organism  of  the  given 
generation.  Solid  black  indicates  the  germ.  Dotted  circles  indicate  gametes 
which  may  perish,  or  may  unite  with  those  of  another  organism. 

process  of  reproduction  is  not  so  directly  associated  with  the 
processes  of  gamete  formation  and  fusion.  Nearly  all  non- 
colonial  Protozoa,  while  in  the  so-called  vegetative  state,  have 
the  power  to  reproduce  by  fission,  so  that  the  plasm  of  these 
cells  is  both  germinal  and  somatic  in  the  Metazoan  sense.  It  is 


16  GENERAL  EMBRYOLOGY 

only  at  irregular  intervals  that  the  individuals  ordinarily 
multiplying  by  fission,  lose  this  property  as  well  as  their 
vegetative  characteristics,  become  specialized  as  gametes,  and 
require  to  undergo  syngamy,  later  resuming  their  duplex 
vegetative  and  reproductive  character. 

While  it  might  be  misleading  to  say  that  the  reproductive 
cells  of  Metazoa  are,  like  the  Protozoa,  both  germ  and  soma,  yet 
it  is  quite  true  that  in  these  germ  cells  we  have  a  substance 
which  produces  both  germinal  and  somatic  tissues.  In  a 
sense  we  are  hardly  j  ustified  in  saying  that  the  soma  is  built  up 
anew  in  each  generation,  while  only  the  germ  has  a  continuous 
existence.  The  germ  cell  is  potentially  soma  as  well  as  germ, 
and  for  a  time  during  the  early  development  of  the  organism 
there  is  no  visible  distinction;  this  distinction  occurs  very  early 
in  the  development  of  a  few  forms,  but  in  most  organisms  not 
until  a  considerable  number  of  cells  has  been  formed.  In 
development  the  germ  cells  give  rise  to  other  cells  like  them- 
selves (germ)  and  to  cells  unlike  themselves  (soma)  and  we 
may  regard  the  "unlike"  as  "new." 

The  common  conception  of  the  life  of  a  species  as  a  succession 
of  generations  of  individuals  linked  together  by  the  germ, 
while  superficially  true,  leads  to  a  fundamentally  erroneous 
point  of  view.  The  fertilized  germ  cell  is  just  as  much  the 
individual  organism  as  the  matured  individual  is.  The  species 
is  no  more  a  succession  of  somas  than  it  is  a  continuous  germ. 
It  is  not  the  function  of  the  germ  to  provide  links  between 
successive  generations  of  "organisms"  or  somas,  any  more  than 
it  is  the  function  of  the  soma  to  insure  the  continuity  of  the 
germ,  and  to  provide  materials  for  its  increase  and  means  of  its 
dispersal.  We  should  recognize  that  the  essential  continuity 
between  successive  generations  is,  after  all,  not  continuity  of 
plasm  but  "continuity  of  organization." 

The  term  reproduction,  strictly  speaking,  does  not  mean 
quite  the  same  thing  among  Metazoa  and  simple  Protozoa. 
Among  the  Protozoa  the  formation  of  free  daughter  cells,  by 
fissions  of  the  zygote  or  its  descendants,  constitutes  repro- 
duction. Among  the  Metazoa  the  corresponding  fissions  of  the 


ONTOGENY  17 

zygote  and  its  daughter  cells  are  not  considered  in  themselves 
reproductive  processes,  but  as  steps  (cell  divisions)  in  the 
building  up  of  the  whole  new  individual,  as  but  one  phase  in 
the  general  process  of  reproduction. 

Some  physiological  reorganization  of  substance,  such  as 
ordinarily  results  from  the  intermingling  of  the  plasmas  of  two 
individuals  or  lines,  seems  a  necessity  for  the  continued  exist- 
ence and  reproductive  activity  of  most  organisms.  In  some 
Insects  (Aphids,  etc.},  Rotifers,  Crustacea,  and  other  forms, 
reproduction  occurs  normally,  through  long  periods,  without 
any  such  syngamic  fusion,  the  new  organisms  developing  from 
single  unfertilized  ova  (parthenogenesis).  While  this  condition 
is,  in  these  cases,  clearly  derived  from  the  normal,  yet  it  seems 
to  illustrate  the  non-essential  relation  of  syngamy  and  repro- 
duction. In  such  forms  syngamy  does  occur  under  certain 
conditions  or  during  certain  periods  in  the  life  cycle  of  the 
organism.  For  example,  the  difficult  conditions  of  winter  or 
drought  may  be  successfully  withstood  by  the  organism  while 
in  the  form  of  the  zygote. 

So  too  in  many,  perhaps  most,  Protozoa,  reproduction  or 
fission  proceeds  normally  and  for  long  periods  without  fertili- 
zation or  conjugation.  The  process  of  conjugation  is  opposed 
to  reproduction  and  may  actually  inhibit  it  for  a  time.  Here 
it  appears  frequently  to  be  associated  with  the  onset  of  con- 
ditions unfavorable  to  the  existence  of  the  organism  in  its 
vegetative  condition. 

It  seems,  therefore,  that  the  processes  of  fertilization  and 
reproduction  may  not  be  essentially  related,  and  that  the 
intermingling  of  the  plasmas  of  two  individuals  is  related 
directly  to  phenomena  other  than  reproduction.  Such  a 
modification  of  substance  as  results  from  fertilization,  however, 
may  be  essential  to  continued  existence,  and  it  is  certainly  true 
of  most  Metazoa  that  such  a  plasmic  fusion  is  an  organic 
necessity.  In  the  simple  Protozoa  this  may  be  accomplished 
at  any  time  by  the  fusion  of  two  individuals  in  the  form  of 
gametes.  In  the  Metazoa,  however,  it  is  obvious  that  this 
necessary  intermingling  of  substance  occurs  only  when  the 


18  GENERAL  EMBRYOLOGY 

organisms  are  in  the  form  of  single  cells,  i.e.,  gametes.  And 
thus  it  comes  about  that  in  the  many-celled  animals,  reproduc- 
tion and  syngamy  are  so  uniformly  associated,  and  while  these 
processes  may  not  have  been  related  primarily,  in  some  in- 
stances are  not  even  at  present,  yet  now  they  have  come  to 
be  so  related  in  the  vast  majority  of  Metazoa,  that  fertilization 
actually  appears  as  the  first  and  most  important  step  in  the 
whole  chain  of  reproductive  events. 

The  actual  processes  involved  in  the  formation,  from  the 
zygote,  of  the  mature  Metazoan  individual  are  extremely 
complicated  and  diverse,  but  they  are  for  the  most  part  reducible 
to  three  fundamental  general  processes.  We  must  leave  aside, 
for  the  present,  the  causal  or  directive  processes  which,  though 
probably  the  essentials  of  development,  are  still  obscure  and 
little  known.  The  grosser  external  phenomena  of  development 
are,  essentially,  growth,  cell  division,  and  differentiation.  The 
living  germ  is  contained  within  the  limits  of  a  single  cell,  often 
of  minute  dimensions  and  only  slightly  differentiated  visibly; 
the  mature  organism  consists  of  an  enormous  number  of  cells, 
comprising  a  considerable  mass,  and  exhibiting  various  degrees 
of  differentiation  in  diverse  directions.  The  transition  from 
one  of  these  states  to  the  other  is  a  gradual  process,  proceeding 
by  minute  steps;  yet  it  is  convenient  to  consider  the  whole  life 
history  of  an  organism  as  a  succession  of  phases,  each  with 
some  chief  characteristic. 

First,  complex  processes  occur  within  the  gametes  or  germ 
cells  themselves,  concerning  chiefly  their  nuclei,  as  a  result  of 
which  they  come  to  have  a  constitution  quite  unlike  that  of  the 
somatic  nuclei.  These  preliminary  events  we  group  under  the 
term  gametogenesis,  or  oogenesis  and  spermatogenesis  in  the  ova 
and  sperm  cells  respectively.  Then  normally  follows  fertiliza- 
tion or  syngamy,  the  fusion  of  the  two  gametes,  derived  from  two 
different  organisms,  into  a  single  cell  which  is  the  "new" 
organism.  Through  fertilization  a  typical  nucleus  is  recon- 
stituted in  the  zygote,  and  there  follows  a  period  of  rapid  cell 
multiplication  which  is  called  the  period  of  segmentation  or 
cleavage.  During  cleavage  are  formed  the  cellular  elements 


ONTOGENY  19 

0 

which  are  to  be  built  into  the  structures  of  the  simple  embryo, 
and  various  differentiated  substances  of  the  egg  are  segregated 
among  different  groups  of  cells.  Following  this  are  the  phases 
of  blastula  formation,  when  the  cells  become  arranged  in  a 
definite  layer,  and  then  gastrula  formation  when  the  cells  are 
rearranged  into  two  definite  layers.  Then  comes  the  period  of 
embryo  formation,  when  the  cells  of  the  layers  are  moulded  into 
the  earliest  beginnings  of  the  chief  systems  and  organs,  blocking 
these  out  in  the  simplest  manner.  During  this  last  phase 
growth  becomes  very  rapid,  accompanied  by  continued  cell 
division,  no  longer  termed  cleavage,  and  as  the  formation  of 
organs  becomes  more  complete  and  more  particular,  the  embryo 
increases  in  bulk  and  dimensions.  This  period  of  embryonic 
development  may  occupy  a  long  time,  and  usually  leads  to 
the  formation  of  an  organism  which  is  capable  of  leading  an 
independent  life,  either  as  a  larva  or  as  a  form  closely  resem- 
bling the  adult,  except  in  size.  Finally,  accompanied  by  con- 
tinued growth,  the  last  phases  of  development  appear  as  cellular 
differentiation  becomes  more  complete,  and  the  organism  begins 
to  assume  more  fully  the  characteristics  of  its  parents.  When 
the  reproductive  tissues  become  functional  as  such,  the  animal 
is  considered  mature  and  its  development  complete,  although 
in  a  true  sense  development  is  never  entirely  completed,  for 
the  form  of  the  organism  never  becomes  definitely  fixed,  and 
cellular  differentiation  seems  never  to  cease  during  the  life  of 
the  organism. 

It  is  important  to  remember  that  all  of  these  phases  of 
development  are  continuous  and  more  or  less  overlapping,  and 
in  all  of  them,  excepting  perhaps  the  earlier,  where  the  impor- 
tant changes  concern  chiefly  the  structure  and  the  composition 
of  the  germ  nuclei,  the  three  processes — growth,  cell  division, 
and  differentiation — are  going  on  together.  Yet  in  general  it  is 
clear  that  in  the  early  stages  of  development,  after  the  gametic 
nuclei  are  differentiated  and  fused,  cell  division  is  the  process 
of  greatest  activity;  then  follow  stages  during  which  develop- 
ment is  characterized,  chiefly  by  growth;  and  lastly  the  final 
aspects  are  chiefly  the  result  of  cellular  or  tissue  differentia- 


20  GENERAL  EMBRYOLOGY 

tion,    processes   often   described   separately  under  the  term 

histogenesis. 

This  brief  outline  reflects  the  fundamental  character  of  the 
relation  of  Embryology,  as  of  all  biological  science,  to  the  Cell 
Theory.  The  recognition  of  the  ovum  (Schwann,  1839; 
Gegenbaur,  1861)  and  spermatozoon  (Schweigger-Seidel,  1865) 
as  modified  cells,  of  the  basic  importance  of  cell  continuity  in 
development  (Virchow),  and  of  the  processes  of  fertilization, 
cleavage,  growth,  and  differentiation  as  essentially  cell  pro- 
cesses, marked  noteworthy  and  fundamental  steps  in  the 
history  of  the  science  of  development.  But  our  recognition 
of  the  importance  of  this  relation,  and  of  the  especial  importance 
of  the  cell  as  the  descriptive  unit  in  development,  should  not 
obscure  the  fact  that  in  many  developmental  processes  we 
cannot  recognize  the  cell  as  the  actual  unit  of  physiological 
activity.  Many  important  steps  in  development  concern  ele- 
ments which  are  distinct  and  individual  intra-cellular  elements. 
And  later,  during  the  cleavage  period,  the  boundaries  of 
specific  materials  behaving  as  units  in  development  do  not 
always  coincide  with  cell  boundaries  or  distributions.  We 
must  regard  the  view  that  the  cells  are  the  ultimate  units  in 
development  as  a  stage  in  the  history  of  opinion,  and  for  the 
present  recognize  certain  intra-cellular  elements  as  the  "ulti- 
mate" structures  in  development. 

But  the  province  of  Embryology  is  not  merely  thus  to  de- 
scribe the  upbuilding  and  unfolding  of  the  structure  and  form 
of  the  new  organism  through  these  successive  stages  of  develop- 
ment; it  is,  further,  to  describe  the  more  fundamental  pro- 
cesses involved  in  this  development,  and  still  further,  to 
summarize  these  descriptions  of  both  kinds  in  the  form  of  simple 
general  statements  or  laws.  In  the  historical  development  of 
the  science  of  Embryology,  as  of  any  natural  science,  the  descrip- 
tion and  comparison  of  visible  forms  and  conditions  came 
first.  This  morphological  account  of  development,  concerned 
chiefly  with  the  description  of  what  happens,  what  is  produced 
in  development,  has  now  been  accomplished  to  such  an  extent 


ONTOGENY  21 

as  to  furnish  a  basis  of  this  kind  sufficient  for  immediate 
necessity.  Next  comes  the  study  of  the  real  processes  leading 
to  the  production  of  one  condition  out  of  another,  processes 
which  underlie  the  externally  visible  form  changes.  This 
physiological  aspect  of  Embryology  is  concerned  more  with 
how  development  occurs,  how,  and  through  the  operation  of 
what  factors  or  mechanisms,  one  condition  leads  to  another. 
In  a  way  this  is  also  the  why  of  development — not  "why"  in 
the  philosophical  sense  of  course,  but  in  the  sense  of  "how 
does  it  happen  that"  these  things  occur  in  development. 
Here  the  two  methods  of  observation  and  experiment  are  com- 
bined and  by  the  artificial  modification  or  the  elimination  of 
one  condition  of  development  after  another,  the  essential 
factors  are  discovered  and  their  modes  of  operation  determined. 
The  science  of  Embryology  has  now  fairly  entered  upon  this 
stage  and  the  dominant  note  of  the  subject  to-day  is  this 
search  for  underlying  processes  and  modes  of  action.  But  as 
yet  it  is  impossible  to  say  that  we  have  reached  the  final 
period  of  the  formulation  of  the  broad  fundamental  generali- 
zations which  give  unity  to  the  infinitely  diverse  phenomena  of 
development,  and  which  are  expressed  in  the  form  of  laws. 
While  something  has  been  accomplished  in  this  direction,  the 
basis  of  fact  is  not  as  yet  sufficiently  broad,  and  the  necessity 
of  frequent  restatement  of  such  "laws"  shows  their  formulations 
to  be  premature,  save  as  guides  in  investigation. 

These  steps  in  the  development  of  the  science  of  Embryology 
do  not  so  nearly  represent  the  course  of  thought  and  hypothesis 
as  that  of  actual  knowledge  and  achievement.  For  even  in 
the  eighteenth  century  the  earliest  embryologists  had  their 
hypotheses  as  to  the  causes  of  this  mysterious  process  of  develop- 
ment. They  offered  first  what  seemed  to  them  an  explanation 
of  the  facts  of  development  which  came  to  be  termed  the  idea 
of  "evolution"  or  preformation.  This  idea  was  that  within 
the  germ,  either  in  the  egg  ("ovists")  or  in  the  spermatozoon 
("spermists, "  "animalculists")  there  was  contained  a  miniature 
organism  resembling,  in  a  general,  or  even  in  a  precise  way, 
the  adult  form  (Fig.  12).  And  this  miniature  had  merely  to 


22 


GENERAL  EMBRYOLOGY 


expand,  or  to  unfold  and  grow,  to  produce  the  individual  of 
the  next  generation.  The  relation  between  the  germ  and  the 
adult  seemed  much  like  that  between  the  bud  and  the  branch  — 
all  the  parts  present  in  minute  rudiments,  ready  to  come  forth 
and  expand.  We  may  recognize  in  this  idea  a  morphological 
conception  of  development  such  as  we  should  expect  to  appear 
first.  This  conception,  with  which  are  associated  such  great 
names  as  Malpighi,  Bonnet,  and  Haller, 
proves  in  reality  an  attempt  to  explain 
development  by  denying  its  occurrence.  For 
the  assumed  formation  of  the  original  indi- 
viduals of  a  species  by  the  Creator  involved 
at  the  same  time  the  creation,  within  them, 
of  the  preformed  germs  of  all  the  other  later 
individuals  of  the  species.  The  belief  that 
the  germ  cells  of  an  organism  contained  in 
miniature  the  members  of  the  second  genera- 
tion necessitated  the  further  belief  that  in 
these  latter  must  be  contained,  within  still 
smaller  limits,  the  individuals  of  the  third 
generation,  and  thus  ad  infinitum.  And  so  it 
was  estimated  that  some  two  hundred  mil- 
lions of  human  beings  were  actually  contained 

FIG.  12.—  Draw-  3  .     J 

ing   of    a    human    in  this  preformed  condition  within  the  ovaries 

sperm  cell  contain-     Q£    Eye        rp^    conception    Qf    infinite    CUCaSC- 
ing  a  miniature  or- 

ganism enclosed  in    ment    or   "  embditement"    proved    to   be   the 

a    thin    membrane.  7       .  .  77  7  fj-iji  «• 

After  o.  Hertwig,   Teductio  ad  absurdum  of  the  theory  of  pre- 
Hartsoeker   formation  nn  this  its  first  and  crudest  form. 


Those  who  actually  observed  the  chick  ap- 
pear within  the  egg  could  not  accept  this  naive  explanation  of 
development,  but  believed  that  there  occurred  a  true  formation 
of  parts  anew  out  of  unformed  material  not  possessing  at  all 
the  characters  of  the  adult  organism.  This  was  Wolff's  idea  of 
epigenesis,  clearly  a  physiological  conception  of  development, 
following  quite  naturally  the  earlier  morphological  conception. 
In  its  original  form  epigenesis  was  chiefly  a  dissent  from  the 
idea  of  preformation  rather  than  an  explanation  of  develop- 


ONTOGENY  23 

ment.  Indeed  it  seems  now  to  have  been  merely  a  restate- 
ment of  the  fact  that  development  occurs,  leaving  this  fact  to 
be  explained  through  the  operation  of  some  supernatural  or 
miraculous  process,  for  the  spontaneous  generation  of  the 
embryo  within  the  egg  was  at  first  definitely  assumed. 

Thus  we  have  almost  from  the  beginning  of  embryological 
study,  two  opposing  explanations  of  the  visible  phenomena 
of  development,  preformation  explaining  development  by  de- 
nying it,  epigenesis  explaining  development  by  reaffirming  it. 
Since  this  early  conflict  of  opinions,  the  crudity  of  which  we  un- 
derstand when  we  think  of  the  means  then  at  hand  for  observing 
such  minute  objects  as  are  many  eggs  and  embryos,  there  has 
been  constant  opposition  of  morphological  and  physiological 
interpretations  of  development.  The  modern  understanding 
of  preformation  is  better  termed  predelineation,  or  better  still, 
predetermination,  less  crude,  less  complete  and  particular  than 
preformation.  What  is  preformed  or  predetermined  in  the 
germ  in  some  way  represents  the  embryo  without  being  at 
all  like  it.  The  idea  of  epigenesis,  too,  is  to-day  less  complete; 
a  certain  structural  organization  is  admittedly  present  in  the 
germ  as  a  heritage  from  previous  generations,  and  real  develop- 
ment occurs  as  a  physiological  process  directed  by  this  rudi- 
mentary structure  already  present.  The  history  of  these  opin- 
ions indicates  that  neither  conception  is  exclusively  true,  but 
that  development  must  involve  both  predetermination  and 
epigenesis;  and  the  present  endeavor  is  to  find  out  not  which, 
but  to  what  extent  each,  is  true. 

The  present  understanding  of  development  seems  to  be  an 
extremely  refined  predetermination  strongly  tinged  with  epi- 
genesis, using  these  words  in  their  modern  sense.  A  more 
extended  statement  of  this  modern  view  and  the  facts  upon 
which  it  is  based  is  reserved  for  a  later  chapter  (Chapter  VII). 
Briefly  stated,  we  believe  that  while  the  embryo,  not  to  say 
adult,  is  by  no  means  preformed  nor  even  fully  predelineated 
in  the  germ,  yet  there  is  a  certain  degree  of  protoplasmic 
structure  or  regional  differentiation  in  the  germ  cells.  This  is 
spoken  of  now  as  the  organization  of  the  germ,  and  it  may  be 


24  GENERAL  EMBRYOLOGY 

both  material  and  dynamic  (i.e.,  energetic).  And  further  this 
organization  is  definitely  related  to  the  structure  of  the  future 
embryo  and  adult,  having  reference,  but  not  resemblance,  to  the 
adult.  The  organization  of  the  cytoplasmic  part  of  the  germ 
is  itself  a  condition  which  develops  (epigenesis)  under  the 
influence  of  the  primary  structure,  or  organization,  of  the 
nucleus.  At  present  this  inherited  organization  or  predelinea- 
tion  of  the  nucleus  seems  primary  and  fixed,  and  to  represent 
the  only  strictly  predelineated  portion  of  the  germ,  controlling 
and  directing  the  later  and  epigenetic  developmental  processes, 
which  may  be  said  often  to  have  commenced  in  the  germ  cells 
even  before  syngamy  has  occurred.  But  history  warns  us 
against  believing  that  this  organization  of  the  nucleus  will 
prove  the  ultimate  organization.  As  knowledge  becomes  more 
complete  this  will  be  thrown  farther  back  to  restricted  elements 
of  the  nucleus;  indeed  it  seems  probable  now  that  the  primary 
organization  concerns,  not  the  entire  nucleus  nor  perhaps  its 
chromosomal  elements  alone,  but  some,  as  yet  invisible, 
problematic,  chemical  and  physical  configurations  of  its 
structure. 

But  we  must  not  search  for  an  explanation  of  the  whole 
process  of  development,  alone  in  the  structure  of  the  germ 
cells.  We  must  look  upon  development  as  upon  other  forms 
of  activity  in  living  things,  as  a  succession  of  reactions  on  the 
part  of  the  organism  to  the  normal  stimuli  of  its  surroundings. 
The  things  that  an  adult  organism  does  are  obviously  reactions; 
it  reacts  to  the  conditions  of  its  environment  by  making  certain 
movements,  forming  certain  substances,  undergoing  certain 
structural  modifications;  in  short,  by  doing  certain  things 
collectively  termed  its  behavior.  The  precise  character  of  an 
animal's  behavior  is  determined  not  alone  by  its  structure, 
by  the  organs  it  has  to  react  with,  nor  alone  by  its  physiological 
condition  at  the  time,  nor  alone  by  the  nature  of  the  external 
conditions  acting;  but  by  all  of  these  combined.  What  the 
adult  organism  does  at  any  particular  moment  is  therefore 
determined  by  two  interacting  sets  of  conditions,  one  within 
the  organism — its  organization,  the  other  without  the  organism 


ONTOGENY  25 

— its  environment.  Either  set  of  conditions  alone  can  lead  to 
no  action;  for  organismal  activity  is  reaction. 

Just  so  the  developing  organism,  at  whatever  stage  it  be 
considered,  reacts  to  the  stimuli  of  its  environment  in  a  manner 
determined  for  the  moment,  on  the  one  hand  by  its  own  state 
or  "organization,"  both  morphological  and  physiological,  and 
on  the  other  by  the  character  of  the  stimuli  acting.  The  ovum 
is  not  to  be  regarded  as  a  mechanism  wound  up,  ready  upon 
receipt  of  a  single  stimulus,  to  go  through  its  development 
into  an  adult  organism.  It  is  rather  to  be  regarded  as  an 
organism  which  reacts  to  its  surroundings  by  undergoing  certain 
changes.  This  changed  organism  then  reacts  further  by  under- 
going certain  other  changes.  One  reaction  of  the  fertilized 
ovum  is  to  cleave,  of  the  blastula  to  gastrulate,  and  so  on. 
Step  by  step,  one  condition  succeeding  another  and  leading  to 
still  another,  the  organism  gradually  alters  its  morphological 
and  physiological  characteristics.  Throughout  its  whole  exist- 
ence the  organism  shows  transformations  of  substance,  energy, 
and  form;  we  agree  to  set  apart  certain  of  these  transformations 
occurring  at  a  very  early  period,  and  to  refer  to  them  as  pro- 
cesses of  " development."  The  normal  " behavior"  of  the  egg 
or  of  the  embryo  is  to  develop.  The  processes  of  development 
are  neither  easier  nor  more  difficult  to  explain  than  the  phe- 
nomena of  adult  behavior,  and  they  have  just  the  same  basis 
in  the  relation  between  the  internal  conditions,  within  the 
organism,  and  the  external  conditions,  without  the  organism, 
at  the  time. 

From  this  point  of  view  the  question  why  the  egg  develops 
is  a  problem  not  different,  in  its  essentials,  from  why  the 
organism  grows,  or  why  it  seeks  or  avoids  the  light.  None  of 
these  is  to  be  solved  by  consideration  of  the  organism  alone, 
whether  egg  or  adult,  apart  from  the  conditions  acting  upon 
the  organism;  both  must  be  studied  together. 

With  this  conception  of  development  in  mind,  we  should  here 
mention  briefly  one  of  the  great  generalizations  that  has  come 
from  the  study  of  organic  development,  namely,  the  Biogenetic 
Law  or  the  Theory  of  Recapitulation.  Briefly  stated  this 


26  GENERAL  EMBRYOLOGY 

familiar  theory  is,  that  the  organism,  in  its  individual  devel- 
opmental history,  tends  to  repeat  in  outline  the  evolutionary 
history  of  its  species.  This  repetition  is  seldom  particular,  or 
detailed,  never  complete,  yet  so  many  of  the  phenomena  of 
development  can  be  satisfactorily  interpreted  from  this  his- 
torical point  of  view,  seeming  to  have  this  historical  significance 
rather  than  an  immediately  adaptive  relation,  that  as  a  general 
statement  the  law  remains  fundamentally  true. 

This  law  is  not  so  much  an  attempt  to  resume  the  facts  of 
embryology  as  to  apply  these  facts  in  the  interpretation  of  racial 
history  (Evolution).  This  application  is  in  many  instances 
difficult  because  of  the  fact  that  there  has  been  an  evolution  of 
the  egg,  the  embryo,  and  the  larva,  just  as  of  the  adult.  The 
fact  that  the  organism  is  specific  at  all  stages  of  its  existence, 
includes  the  parallel  evolution  of  ova,  and  of  all  succeeding 

Species        Zygote  Stages  in  Development  Adult 

D  AD  BD  CD  D 

E  AE  BE  CE  DE  E 

F  AF  BF  CF  DF  EF  F 

G  AG         BG         CG         Do         Ea          FG         G 

H  AH        Bu        CH        DH        EH        FH        GH        H 

FIG.  13. — Diagram  to  illustrate  the  essentials  of  the  Biogenetic  Law.      Modified 

from  O.  Hertwig. 

developmental  stages,  if  there  is  to  be  any  evolution  of  adult 
structures,  else  diversity  of  adult  organization  would  depend 
upon  external  conditions  of  development,  rather  than  upon  egg 
organization.  But  we  know  that  the  eggs  of  any  species  of 
sea-urchin  and  star-fish  will  develop,  respectively,  into  adult 
sea-urchins  and  star-fish  of  those  species,  although  in  the  same 
dish,  with  identical  environing  stimuli. 

Many  important  points  concerning  the  relation  between 
ontogeny  and  phylogeny  may  be  represented  schematically, 
as  in  the  accompanying  diagram  (Fig.  13).  Here  we  com- 


ONTOGENY  27 

pare  the  ontogenies  of  five  related  species,  the  adults  of  which 
represent  an  evolutionary  series;  species  E  has  evolved  beyond 
D,  F  beyond  E,  and  so  forth.  The  first  stage  (i.e.,  the 
fertilized  ovum  or  zygote)  of  D  is  not  merely  a  zygote  (A), 
but  it  is  the  zygote  of  species  D,  and  consequently  indicated  in 
our  diagram  by  AD;  in  its  development  this  passes  through 
the  specific  stages  BD,  and  CD,  to  the  adult  D.  Species  E 
is  more  highly  evolved  than  species  D,  but  it  begins  its  existence 
as  a  fertilized  ovum  which  again  is  specific,  this  time  AE.  In 
its  development  to  the  adult  form  this  may  pass  through  stages 
B  and  C  similar  to  those  of  species  D,  but  merely  similar,  not 
identical,  else  the  result  would  be  D  and  not  E.  Therefore,  we 
call  these  intermediate  stages  BE  and  CE.  Further,  species  E 
may  pass  through  a  stage  in  some  particulars  resembling  D; 
this,  however,  does  not  exactly  resemble  D  and  is  therefore 
designated  DE.  Similarly  for  species  F,  G,  and  H;  each  is 
more  highly  evolved  than  the  preceding.  Each  passes  through 
stages  which  resemble  stages  in  the  development  of  the  less 
highly  evolved  species,  yet  each  stage  is  really  specific. 

Conditions  of  life  change  for  the  embryo  as  well  as  for  the 
adult,  and  if  these  younger  organisms  are  to  remain  in  existence 
they  must  evolve  to  meet  the  changed  conditions.  The  process 
of  evolution  concerns  not  merely  the  adult,  but  the  organism 
at  every  stage  of  its  existence.  Stages  such  as  BG  or  CH  may 
finally  become  so  highly  modified  that  they  are  no  longer  recog- 
nizable as  related  to  B  and  C,  and  might  as  well  be  termed 
XG  and  YH.  It  is  then  said  that  these  traits  are  "  coenogenetic 
modifications"  in  distinction  from  " palingenetic  characteris- 
tics," which  are  obvious  similarities  to  previous  racial  condi- 
tions. But  recognition  of  the  idea  that  the  entire  life  history 
is  undergoing  evolution,  at  every  point,  very  largely  minimizes 
the  value  of  this  very  common  distinction  between  ccenogenetic 
and  palingenetic  traits  in  development,  for  in  a  very  true  sense 
all  the  traits  of  the  developing  organisms  are  in  varying  degrees 
both  coenogenetic  and  palingenetic. 

Finally,  we  see  that  the  problem  why  the  egg  develops  into 
a  form  resembling  its  progenitors,  rather  than  organisms  of 


28  GENERAL  EMBRYOLOGY 

another  kind,  that  is  to  say,  the  problem  of  heredity,  may  be 
more  clearly  understood  by  recognizing  that  the  characteristics 
of  the  organism  are  specific  at  all  stages  of  its  existence.  The 
egg  of  the  star-fish  is  just  as  much  a  star-fish  as  the  adult  is.  The 
germinal  substance  of  successive  generations  of  star-fish  is 
directly  continuous.  This  continuity  of  specific  organization 
through  the  germ,  combined  with  essentially  uniform  conditions 
of  development,  determines  the  essential  uniformity  of  each 
series  of  interactions  leading  to  the  formation  of  a  new  adult 
organism.  In  a  real  sense  the  problem  of  heredity  thus  becomes 
the  same  as  the  problem  of  development.  And  the  problem 
why  the  egg  of  the  star-fish  develops  into  a  star-fish  and  not 
into  a  sea-urchin,  is  fundamentally  the  same  as  the  problem 
why  the  star-fish  is  not  a  sea-urchin;  it  is  the  general  problem 
of  the  evolution  of  organic  diversity. 

REFERENCES  TO  LITERATURE 

In  the  "references  to  literature,"  given  at  the  end  of  each  chapter, 
the  author's  name  and  the  title  of  the  work,  are  followed  by  the  reference 
to  the  journal  in  which  the  work  appeared,  or  to  the  place  of  publication, 
in  case  the  work  is  a  separate  publication.  The  number  of  the  volume 
(Band,  tome,  etc.)  is  printed  in  black-face  Arabic  numerals,  followed 
by  the  year  of  appearance.  References  to  pages,  parts,  etc.,  are  omitted 
except  in  a  few  necessary  instances. 

The  abbreviations  of  the  more  common  references  are  as  follows : 
Amer.  Jour.  Anat.    American  Journal  of  Anatomy.     Baltimore  and 

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lungsgeschichte .     Bonn. 


ONTOGENY  29 

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Q.  J.  M.  S.     Quarterly  Journal  of  Microscopical  Science.     London. 
Sitz.-Ber.  Acad.  Wiss,  Berlin.     Sitzungsberichte  der  koniglich  preussis- 

chen  Akademie  der  Wissenschaften  zu  Berlin. 
Sitz.-Ber.  Phys.-Med.  Ges.  Wtirzburg.     Sitzungsberichte  der  Physicalisch- 

medizinisch  Gesellschaft  zu  Wiirzburg. 
Sitz.-Ber.    Ges.    Morph.    Phys.     Sitzungsberichte    der    Gesellschaft  fur 

Morphologic  und  Physiologic  in  Miinchen. 
Trans.   Am.   Phil.   Soc.     Transactions   of  the   American   Philosophical 

Society.     Philadelphia. 
Zeit.    Indukt.   Abstamm.    Vererbungslehre.     Zeitschrift  fur   induktive 

A bsta mmun gs-  un d  Vererbungslehre.     Berlin. 

Zeit.  wiss.  Zool.     Zeitschrift  fiir  u'issenschaftliche  Zoologie.     Leipzig. 
Zool.  Jahrb.     Zoologische  Jahrbiicher.     (Abteilung  fur  Anatomic  und 

Ontogenie  der  Tiere,  unless  otherwise  specified.)     Jena. 


30  GENERAL  EMBRYOLOGY 

REFERENCES  TO  LITERATURE— CHAPTER  I 

CALKINS,  G.  N.,  The  Protozoa.     Columbia  Univ.  Biol.  Ser.  VI.     New 
York.     1901. 
Protozoology.     New  York.     1909. 

DOFLEIN,  F.,  Lehrbuch  der  Protozoenkunde.     Jena.     (3  Aufl.)     1911. 

HARTSOEKER,  N.,  Essay  de  dioptrique.     Paris.     1694. 

HERTWIG,  O.,  Zeit-  und  Streitfragen  der  Biologie.  I.  Pra  formation 
oder  Epigenese.  Grundziige  einer  Entwickelungstheorie  der 
Organismen.  Jena.  1894.  English  translation  by  P.  C.  Mitchell, 
"The  Biological  Problem  of  To-day:  Preformation  or  Epigenesis," 
etc.  London.  1896.  Die  Entwickelungslehre  im  16.  bis  18. 
Jahrhundert.  Handbuch  der  vergl.  u.  exp.  Entwick.  d.  Wirbelt. 
I,  1,  1.  Jena.  1906.  (1901.)  Ueber  die  Stellung  der  vergleich- 
enden  Entwickelungslehre  zur  vergleichenden  Anatomie,  zur 
Systematik  und  Descendenztheorie.  (Das  biogenetische  Grund- 
gesetz,  Palingenese  und  Cenogenese.)  Id.  Ill,  3.  1906. 

HERTWIG,  R.,  Mit  welchem  Rechte  unterscheidet  man  geschlechtliche 
und  ungeschlechtliche  Fortpflanzung?  Sitz.-Ber.  Ges.  Morph. 
Phys.  15.  1899.  English  translation  by  Curtis,  W.  C.,  Science. 
12.  1900. 

KOFOID,  C.  A.,  On  Pleodorina  illinoisensis,  a  new  Species  from  the 
Plankton  of  the  Illinois  River.  Bull.  111.  State  Lab.  Nat.  Hist.  5. 
1898. 

KORSCHELT  und  HEIDER,  Lehrbuch  der  vergl.  Entwickelungsgeschichte 
der  wirbellosen  Thiere.  IV  Abschnitt.  Ungeschlechtliche  Fort- 
pflanzung and  Regeneration.  Jena.  1910. 

LILLJE,  F.  R.,  The  Theory  of  Individual  Development.  Pop.  Sci.  Mo. 
75.  1909. 

LOCY,  W.  A.,  Biology  and  its  Makers.     New  York.     1908. 

MORGAN,  T.  H.,  The  Problem  of  Development.  International  Monthly. 
Burlington,  Vt.  1901. 

SCHULTZ,  E.,  Prinzipien  der  rationellen  vergleichenden  Embryologie. 
Leipzig.  1910. 

WEISMANN,  A.,  Essays  on  Heredity.  English  Translation.  I  and  II 
Series.  Oxford.  1891,  1892.  The  Germ  Plasm.  English  Trans- 
lation. New  York.  1893.  The  Evolution  Theory.  English 
Translation.  New  York.  1904. 

WHITMAN,  C.  O.,  The  Inadequacy  of  the  Cell-theory  of  Development. 
Woods  Holl  Biol.  Lect.  1893.  Bonnet's  Theory  of  Evolution. 
Id.  1894.  Evolution  and  Epigenesis.  Id.  1894.- 

WILSON,  E.  B.,  The  Problem  of  Development.    Science.    21.     1905. 


CHAPTER  II 

THE  CELL  AND  CELL  DIVISION 

Two  universal  characteristics  of  living  things  are  the  posses- 
sion of  protoplasm  and  a  cellular  composition.  Recognition  of 
these  fundamental  facts  was  dependent  upon  the  use  of  the 
compound  microscope,  and  so  we  find  them  comparatively 
late  acquirements  in  the  history  of  biology.  The  cell-unit 
structure  of  an  organic  tissue  was  first  described  in  plants 
(cork  tissue),  by  Robert  Hooke  in  1665,  and  quite  naturally, 
therefore,  emphasis  was  laid  upon  what  we  now  know  as  the 
''cell  walls."  As  a  consequence  the  term  "cell"  was  applied 
to  these  small  box-like  units  which  seemed  to  resemble  the 
cells  of  a  honey  comb.  When,  about  the  middle  of  the  nine- 
teenth century,  it  became  apparent  that  the  cell  content,  and 
not  the  cell  wall,  was  the  important  thing,  the  word  "cell"  had 
become  so  definitely  fixed  that  it  could  not  but  be  retained, 
although  its  utter  inaptness  was  fully  recognized. 

This  is  not  the  place  to  discuss  the  general  importance  and 
significance  of  the  Cell  Theory  of  Schleiden  and  Schwann  (1839) 
and  their  successors.  It  will  become  clear  as  we  proceed  that 
the  cell,  in  structure  and  in  action,  is  the  basis,  of  modern 
Embryology.  Most  of  the  early  processes  of  organic  develop- 
ment are  strictly  cell  processes  and  must  be  studied  from  the 
standpoints  of  both  Cytology  and  Embryology,  from  neither 
alone,  and  throughout  development  constant  reference  must 
be  had  to  cellular  phenomena. 

As  known  to-day  cells  of  different  organisms  and  different 
tissues  exhibit  an  unending  variety  in  size,  form,  structure,  and 
function  (Figs.  14,  15),  but  throughout  there  are  two  essentials 
of  structure  expressed  by  the  definition  of  a  cell  given  by  Leydig 
(1852)  and  by  Schultze  (1861)  as  "a  mass  of  protoplasm  con- 

31 


32 


GENERAL  EMBRYOLOGY 


VIII 


"| 

' 


FIG.  14. — Various  forms  of 'cells.  IV-IX,  from  Dahlgren  and  Kepner,  X, 
after  Prenant  and  Bouin.  /,  II.  Human  leucocyte,  X  350.  ///.  Human  red 
blood-corpuscle,  X  350.  IV.  Cell  from  root  cap  of  calla  lily,  X  350;  p,  plastids. 
V.  Epidermis  of  earthworm,  showing  four  mucous  cells  in  various  stages  of 
secretion,  and  CM,  cuticle,  X  550.  VI.  Fat  cells  in  skin  of  chicken;  n,  nucleus. 
X  435.  VII.  Ovarian  ovum  (oocyte)  of  cat;  cm,  cell  membrane;  mi,  micro- 
somes;  nm,  nuclear  membrane;  o,  nucleolus;  y,  yolk  alveoli.  VIII.  Connective 
tissue  cells  from  the  lobster;  cy,  cytoplasmic  mass;  pr,  cytoplasmic  processes; 
per,  peripheral  layer  of  cytoplasm  upon  which  the  rigid  material  of  the  tissue  is 
laid  down.  IX.  Pigment  cell  from  the  peritoneum  of  the  fish,  Ammodytes. 
Fully  extended;  the  processes  can  be  completely  retracted.  Two  nuclei,  X  90. 
X.  Stratified  epithelium  from  human  pharynx,  showing  intercellular  connections 
or  bridges,  X  375. 


THE  CELL  AND  CELL  DIVISION 


33 


XVI 


FIG.  15. — Various  forms  of  cells,  continued.  XI-XIII,  after  Prenant  and 
Bouin,  XIV-XVI,  from  Dahlgren  and  Kepner.  XI.  Ganglion  cell  from  human 
spinal  cord;  n,  nucleus,  X  250.  XII.  Sensory  (receptor)  cells  from  human 
olfactory  epithelium,  X  175.  XIII.  Multipolar  ganglion  cell  from  optic  ganglion 
of  horse.  (The  processes  of  the  right  side  are  cut  off.)  a.  Axon,  X  63.  XIV. 
Part  of  muscle  cell  from  the  fish,  Catostomus;  c,  capillary  containing  blood  cells 
and  platelets,  X  500.  XV.  Ciliated  cells  from  the  digestive  tract  of  the  Mollusc, 
Cyclas.  XVI.  Gland  cell  from  the  leech,  Piscicola;  ch,  cytoplasmic  channels 
containing  the  secretion  granules;  dt,  discharging  tubes;  n,  nucleus;  o,  nucleolus; 
s,  secreted  materials  in  various  stages  of  elaboration. 


34  GENERAL  EMBRYOLOGY 

taming  a  nucleus."  To-day  we  modify  this  but  slightly  and 
define  a  cell  as  a  limited  mass  of  protoplasm  containing  nuclear 
material. 

The  mass  of  the  cell  is  usually  very  small.  The  smallest 
cells  known  are  the  Bacteria,  some  of  which  may  be  only 
0.001  mm.  (1  micron,  or  1/25000  inch)  in  length  (Streptococci), 
or  even  less  (Staphylococci).  But  among  the  Metazoa,  cells 
are  never  so  minute.  The  human  white  blood  corpuscle  or 
leucocyte  (Fig.  14,  7),  is  perhaps  of  average  size,  measuring 
something  less  than  0.01  mm.  (8-10  micro)  in  diameter. 
Tissue  cells  having  a  diameter  of  0.05  mm.  (50  micro)  are 
considered  large,  although  a  few  specialized  cells  may  far  exceed 
this  (e.g.,  muscle  or  nerve  cells).  The  egg  cells  of  animals  are 
usually  larger  than  tissue  cells,  but  this  is  a  special  condition, 
and  is  frequently  due  to  the  accumulation  of  stored  food  sub- 
stance, rather  than  to  the  possession  of  a  larger  amount  of 
protoplasm.  Within  the  species  the  sizes  of  specific  varieties 
of  cells  are  very  constant.  The  size  of  an  organ  or  of  an 
individual  is  related  to  the  number  of  its  component  cells 
rather  than  to  their  size  (Amelung,  Conklin). 

We  may  proceed  now  to  describe  the  essentials  of  structure 
exhibited  by  a  typical  cell — an  imaginary  thing  which  has  no 
more  real  existence  than  the  "  average  man."  Such  a  cell 
would  consist  of  a  spheroidal  or  irregular  mass  of  protoplasm, 
limited  by  a  definite  cell  membrane  or  cell  wall.  The  wall  may 
be  a  surface  condensation  of  the  protoplasm  or,  more  frequently, 
a  true  secretion  of  the  cell  body,  either  membranous  as  in  most 
animal  tissues,  or  thick  and  rigid,  like  the  cellulose  walls  of 
most  plants.  In  many  cells  the  viscid  transparent  protoplasm 
just  within  and  in  contact  with  the  cell  wall  forms  a  thin  layer, 
the  ectosarc  or  ectoplasm  (Fig.  16),  clearer  than  the  granular 
and  more  refractive  central  endosarc  or  endoplasm,  which 
contains,  besides  the  granules,  many  cell  organs  and  inclusions. 

The  protoplasm  itself  is  made  up  of  a  combination  of  two 
forms,  perhaps  two  kinds,  of  material  plainly  differing  in  density 
and  arrangement  (Fig.  17).  The  denser  material  called  the 
mitome,  spongioplasm,  reticulum,  or  filar  substance,  forms  a  sort 


THE  CELL  AND  CELL  DIVISION 


35 


of  complex  framework  or  fine  network  of  irregularly  woven 
paths  along  which  are  scattered  minute  granules  called  micro- 
somes.  The  spaces  or  meshes  of  this  spongioplasmic  network 
are  filled  with  the  less  dense  ground  substance  or  cell  sap,  called 
also  the  hyaloplasm,  paraplasm,  or  inter  filar  substance.  The 


en 


FIG.  16. — Diagram  of  a  typical  cell,  a,  aster;  c,  centrosome  (centriole) ; 
ch,  chromatin;  cr,  chromidia;  cs,  centrosphere ;  d,  deutoplasmic  granules;  en, 
endoplasm;  ex,  exoplasm  (cortical  plasm);  hy,  hyaloplasm;  k,  karyosome;  I,  linin 
network;  m,  cell  membrane;  n,  nucleus;  nra,  nuclear  membrane;  o,  nucleolus; 
p,  plastids;  sp,  spongioplasm;  v,  fluid  vacuoles  (metaplasm). 

actual  relation  of  these  two  kinds  of  substances  varies  in 
different  kinds  of  cells  or  even  at  different  times  in  the  same 
cell.  A  frequent  arrangement  is  that  of  a  reticulum  just 
described,  in  which  the  spongioplasm  is  definitely  fibrous, 
forming  a  felt-work  holding  the  more  fluid  hyaloplasm.  In 
other  cells,  or  at  other  times,  protoplasm  has  a  distinctly  alveolar 
structure  resembling  a  fine  emulsion.  Here  the  hyaloplasm  is 


36 


GENERAL  EMBRYOLOGY 


in  the  form  of  minute  drops  or  alveoli,  while  their  walls  or  the 
irregular  interalveolar  spaces  are  of  the  denser  material. 
Occasionally  other  structural  relations  are  seen,  such  as  the 
granular,  where  the  fibrous  reticulum  is  represented  by  rows  of 
excessively  minute  granules,  and  the  fibrillar,  where  the  fibers 


FIG.  17. — Alveolar  protoplasmic  structure  in  the  egg  of  the  sea-urchin,  Toxo- 
pneustes,  one  and  one-half  minutes  after  the  entrance  of  the  spermatozoon. 
From  Wilson,  "Cell,"  X  about  2000.  The  protoplasm  consists  of  alveoli  sur- 
rounded by  microsomes.  In  the  middle  is  the  centriole,  surrounded  by  the 
centrosphere,  while  radiating  from  it  are  the  rays  of  the  aster.  The  large  and 
small  black  masses  are  the  sperm  head  and  middle-piece. 

of  the  reticulum  are  larger,  longer,  and  less  branched  than  in 
the  ordinary  reticulum.  It  is  still  uncertain  how  exactly  the 
real  structure  of  living  protoplasm  is  represented  by  its  appear- 
ance after  it  has  been  killed,  in  preparing  it  for  examination. 
It  should  be  remembered  that  in  living  protoplasm  these 
fibrils,  reticula,  etc.,  are  in  all  probability  fluid  structures  of 
greater  density  than  the  ground  substance. 

The  cell  is  far  from  being  a  simple  unit,  for  it  contains  a 


THE  CELL  AND  CELL  DIVISION  37 

variety  of  structures  and  materials  differing  chemically  and 
functionally;  these  may  not  all  be  directly  visible  as  organized 
structures.  The  only  constantly  differentiated  substance  is  the 
nuclear  material  which  is  usually  contained  in  a  definitely 
formed  body,  the  nucleus,  though  it  may  be  scattered  through 
the  protoplasm.  There  are  many  reasons  for  believing  that 
primitively  the  nuclear  substance  was  not  thus  organized  into 
a  definite  nucleus,  but  that  it  was  distributed  through  the 
cytoplasm  in  the  form  of  small  granules,  as  it  is  still  in  many 
of  the  simplest  organisms,  and  that  gradually  these  became 
aggregated  into  fewer  larger  masses.  The  Protozoa  show  many 
stages  in  the  gradual  enlargement  and  numerical  reduction  of 
the  nuclear  elements,  but  in  the  Metazoa  the  nuclear  material 
is  nearly  always  collected  into  a  single  body.  The  nucleus  is 
to  be  regarded  as  a  specialized  portion  of  the  protoplasm  of 
the  cell,  highly  differentiated  in  structure,  chemical  compo- 
sition, function,  and  behavior.  All  cell  activities  seem  to 
involve  mutual  interaction  between  the  nucleus  and  the 
remainder  of  the  cell  and  neither  is  able  long  to  function 
normally  without  the  other.  But  the  action  of  the  nucleus  is 
primary  and  directive,  to  a  large  extent  controlling  and  regu- 
lating cell  activities  and  cell  life  as  a  whole.  In  most  cases 
the  nucleus  is  a  spherical  or  ovoid  body  of  fixed  form;  in  some 
very  active  cells  it  may  be  elongated  or  of  irregular  form,  or 
even  branched,  ramifying  all  through  the  cell;  in  a  few  rare 
instances  the  nucleus  may  be  amoeboid  (Figs.  14, 15). 

Typically  this  complex  center  of  cell  activity  shows  much  the 
same  fundamental  structure  as  the  remainder  of  the  protoplasm, 
which  in  distinction  from  the  nucleus  is  called  the  cytoplasm. 
The  nucleus  is  limited  by  a  definite  nuclear  wall  or  membrane 
formed  either  from  the  cytoplasm  or  from  the  nucleus  itself. 
The  substance  of  the  nucleus  as  a  whole  is  termed  the  karyo- 
plasm.  The  equivalent  of  the  spongioplasmic  reticulum  of  the 
cytoplasm  is  here  termed  the  linin  network,  and  the  hyalo- 
plasmic  ground  substance  is  known  as  the  nuclear  sap,  or 
karyolymph,  or  paralinin.  The  chief  distinction  of  the  nucleus 
is  the  presence  of  a  very  special  nucleo-protein  substance  called 


38  GENERAL  EMBRYOLOGY 

chromatin,  a  name  given  it  on  account  of  the  ease  with  which 
it  is  colored  with  such  dyes  as  hsematoxylin  (logwood)  or 
carmine.  This  chromatin  is  ordinarily  in  the  form  of  small 
flakes  or  granules  of  variable  size,  termed  chromioles  (Eisen). 
These  are  always  distributed  along  the  linin  fibers,  which  stain 
less  readily  and  are  therefore  described  as  achromatic. .  Some- 
times a  few  large  masses  of  chromatin  called  karyosomes  may 
be  seen  within  the  nucleus.  Apparently  the  chromatin  may 
be  rapidly  dissolved  or  condensed,  or  even  formed  anew,  so 
that  its  visible  amount  and  condition  vary  considerably  from 
time  to  time. 

The  structural  differentiation  of  the  nucleus  is  frequently 
complicated  further  by  the  presence  of  one  or  more  bodies 
called  plasmosomes  or  nucleoli,  which  often  superficially  resem- 
ble the  karyosomes,  at  least  morphologically,  although  chemic- 
ally and  physiologically  they  are  quite  unlike  (Fig.  16).  The 
nucleoli  are  spheroidal  bodies,  staining  densely,  though  not  of 
the  same  material  as  the  true  chromatin/  The  karyosomes  are 
sometimes  called  chromatin  nucleoli.  The  nucleoli  vary  con- 
siderably in  number,  size,  and  form,  and  their  significance  in 
the  nucleus  is  not  altogether  understood;  probably  several 
unlike  bodies  having  different  functions  have  been  included 
under  this  single  term. 

Another  organ  of  the  cell  is  the  centrosome.  This  is  not 
always  to  be  seen  for  it  may  be  lost  from  the  cell  at  certain 
times  and  reappear  later.  While  commonly  a  cytoplasmic 
structure  lying  just  outside  the  nucleus,  in  some  forms  it  is 
an  intra-nuclear  organ  and  it  is  quite  possible  that  primitively 
it  was  an  essential  part  of  the  nucleus,  as  it  still  is  in  many 
Protozoa  (Figs.  29,  30).  The  centrosome  is  a  minute  densely 
staining  granule  or  pair  of  granules,  sometimes  hardly  larger 
than  the  granules  or  microsomes  of  the  cytoplasmic  reticulum. 
Occasionally  it  consists  of  several  separate  granules  closely 
associated.  The  cytoplasm  in  the  neighborhood  of  the  centro- 
some and  directly  under  its  influence  is  ordinarily  differentiated 
as  the  archoplasm.  This  substance  consists  typically  of  two 
portions.  A  medullary  region  forming  a  small  spheroidal 


THE  CELL  AND  CELL  DIVISION  39 

mass  called  the  centrosphere  or  attraction  sphere  immediately 
surrounds  the  centrosome,  and  peripherally,  radiating  from 
this  out  into  the  cytoplasm,  there  is  at  times  a  collection  of 
diverging  rays  or  fibers  called  collectively  the  aster.  In  the 
ordinary  vegetative  cell  the  centrosome  and  archoplasmic 
structures  are  usually  reduced  in  size  and  perhaps  even  absent 
in  some  cells,  but  in  the  dividing  cell  they  may  be  very  large 
and  prominent  organs  extending  nearly  throughout  the  cell, 
for  their  chief  function  is  in  connection  with  the  process  of 
cell  division.  Similar  dense  granules  and  fibers  are  found  as- 
sociated with  organs  of  the  cell  which  are  motile,  for  example, 
the  granules  at  the  bases  of  cilia  and  flagella,  the  axial  fila- 
ments of  some  flagella,  undulating  membranes,  and  some 
pseudopodia,  the  fibrillse  of  muscle  cells,  etc.  All  such  modi- 
fications of  protoplasm  connected  particularly  with  the  pro- 
duction or  regulation  of  motion  are  included  under  the  general 
term  kinoplasm  (Strasburger) .  It  has  been  suggested  that 
the  kinoplasm  of  the  cell  is  of  the  nature  of  a  definite  and 
permanent  cell  organ.  In  many  cells  the  centrosome  is  such 
a  permanent  organ,  but  many  other  kinoplasmic  structures 
seem  to  be  more  or  less  temporary  and  may  disappear  or 
be  formed  anew  at  the  time  of  cell  division  or  at  other  times. 
The  rays  of  the  aster,  for  example,  in  many  cases  apparently 
are  formed  from  the  enlargement  and  rearrangement  of  the 
cytoplasmic  reticulum  resulting  from  the  activity  (probably 
chemical)  of  the  centrosome. 

In  addition  to  these  nearly  constant  cell  structures  of  com- 
paratively uniform  characteristics,  there  are  various  other 
organs  or  bodies  which  are  peculiar  to  certain  special  kinds 
of  cells.  Among  these  are  the  bodies  called  in  general  plastids 
(Fig.  14,  IV),  of  which  the  more  familiar  are  the  pigment  bodies, 
such  as  chloroplastids  or  chlorophyl  bodies,  the  chromoplastids 
or  colored  bodies  not  containing  chlorophyl,  amyloplastids  or 
starch-forming  bodies,  protein-forming  plastids,  and  others. 
Vacuoles  of  various  sizes  and  kinds  are  very  common,  such  as 
the  digestive,  excretory,  food,  water,  and  food-storage  vacuoles; 
occasionally  one  or  more  are  specialized  as  contractile  or  pul- 


40 


GENERAL  EMBRYOLOGY 


sating  vacuoles.  Nutritive  substances  are  often  stored  tem- 
porarily in  cells.  These  materials  are  collectively  known  as 
deutoplasm,  metaplasm,  or  paraplasm,  and  may  be  starch  or 
protein  granules,  yolk  plates,  oil  drops,  etc.  There  are  also 


n. 


FIG.  18. — "Trophospongien"   in   cells   of  the  hepatic   duct  of  the  snail,  Helix 
pomatia.     After  Erhard.     g,  basal  granules;  n,  nucleus;  t,  trophospongien. 

granules  of  various  other  kinds — of  materials  being  secreted 
or  excreted,  of  food  in  various  stages  of  ingestion  or  digestion, 
pigment  granules,  crystals,  and  formed  substances  of  many 
kinds,  usually  specific  for  the  organism  or  tissue. 

In  many  cells  true  chromatic  structures  are  found  in  the 


THE  CELL  AND  CELL  DIVISION  41 

cytoplasm  outside  the  definite  nucleus.  These  are  usually 
small  granules  and  bits  of  chromatin  of  varied  form  and 
significance  in  different  cells.  Collectively  they  are  termed 
chromidia;  for  the  most  part  they  are  not  formed  in  situ,  but 
are  derived  directly  from  the  nucleus  (Fig.  18).  They  are  most 
frequent  in  very  active  cells  such  as  gland  cells,  the  rapidly 
growing  germ  cells,  and  many  others.  Many  forms  of  chro- 
midia, functionally  as  well  as  structurally  distinct,  have  been 
given  special  names  such  as  chondriosomes,  "Chondromiten," 
idiochromidia,  mitochondria,  pseudochromosomes,  "  Nebenkern," 
"  Trophospongien"  etc. 

It  has  been  said  already  that  no  single  cell  shows  typically 
all  the  structures  described  above  and  illustrated  in  Fig.  16. 
Without  stopping  for  further  description  many  of  the  details 
of  structure  truly  characteristic  of  a  few  varied  types  of  cells 
are  shown  for  comparison  in  Figs.  14,  15.  (For  descriptions  and 
further  details  the  student  should  consult  the  standard  texts 
of  Cytology  and  Histology,  e.g.,  Dahlgren  and  Kepner's 
''Principles  of  Animal  Histology/'  New  York,  1908.) 

One  further  aspect  of  cell  structure  remains  to  be  mentioned. 
This  is  the  important  fact  that  there  is  in  most  tissues  a  fairly 
definite  cell  form  combined  with  a  nearly  constant  arrangement 
of  the  cell  organs  and  contents  (Van  Beneden,  Rabl,  Heidenhain). 
In  any  epithelial  cell  the  different  physiological  conditions  at 
the  free  and  the  attached  surfaces  lead  to  this  definite  relation 
of  cell  structures  which  is  called  polarity.  In  such  cells  the 
nucleus  lies  toward  the  basal  end  of  the  cell,  the  centrosome 
either  toward  the  free  end  or  on  that  side  of  the  nucleus.  Thus 
an  imaginary  axis  may  be  passed  through  the  centrosome  and 
nucleus  perpendicular  to  the  free  surface,  about  which  the 
cell  organs  are  arranged  symmetrically,  either  bilaterally  or 
radially  (rotatorially ) .  This  polarity  extends  also  to  other 
less  constant  structures  and  even  occasionally  to  non-epithelial 
cells.  It  may  be  seen  for  example  in  the  arrangement  of  cilia, 
cuticle,  conductile  and  contractile  fibers,  granules  or  drops  of 
substances  being  excreted  or  secreted,  and  yolk  or  other  stored 
materials  (Figs.  14,  15,  16,  etc.).  In  the  germ  cells,  as  we  shall 


42  GENERAL  EMBRYOLOGY 

see  later,  this  fact  of  polarity  becomes  of  the  greatest  importance, 
for  it  is  frequently  related  closely  to  the  symmetry  of  the  mature 
organism. 

While  we  may  thus  describe  the  Metazoan  tissue  cell  as  a 
separate  morphological  unit,  complete  in  itself,  it  is  also  true 
that  in  a  great  many  tissues  the  cells  are  in  direct  material 
continuity  with  one  another  through  minute  protoplasmic 
connections  or  bridges  which  pass  through  fine  perforations  in 
the  cell  walls.  These  have  been  observed  in  a  great  variety 
of  tissues  in  many  different  forms  (Fig.  14,  X) ;  whether  or  not 
they  are  present  in  all  tissues  it  is  yet  impossible  to  say  definitely, 
and  we  must  recognize  clearly  that  in  the  physiology  of  the 
organism  the  cells  do  not  behave  as  completely  autonomic 
units.  While  each  represents  a  localized  field  of  activity,  the 
life  of  the  cell  is  subordinated  to  the  life  of  the  organism  as  a 
whole — a  fact  that  comes  out  with  especial  clearness  in  the 
development  of  the  organism.  In  some  way  not  yet  understood 
the  cell,  or  groups  of  cells,  influence  the  activities  of  other  cells, 
and  are  in  turn  influenced  by  them.  The  activities  of  the 
Metazoan  organism  of  course  equal  the  sum  of  the  activities  of 
its  component  cells;  but  these  combined  activities  are  organized 
and  unified  into  a  whole  in  such  a  way  that  this  represents 
more  than  the  unit  activities  when  considered  separately,  just 
as  the  action  of  a  community  represents  something  beyond 
the  sum  of  the  actions  of  its  members  taken  individually. 

From  the  embryological  point  of  view  one  of  the  most  im- 
portant phases  of  cell  activity  is  cell  reproduction  or  cell 
division.  For  cells  arise  only  from  preexisting  cells.  The 
history  of  opinion  regarding  the  genesis  of  cells  parallels  roughly 
that  regarding  the  genesis  of  organisms.  It  was  Virchow  who 
finally  demonstrated  convincingly  (1855,  1858)  the  universality 
of  the  fact  of  cell  continuity  in  the  tissues  of  a  single  organism, 
and  further  the  fact  that  in  a  succession  of  generations  of 
organisms  the  process  of  the  formation  of  cells  from  preexisting 
cells  is  not  interrupted.  We  know  now  that  in  this  process  of 
cell  division  all  the  essential  organs  of  the  cell  take  an  active 


THE  CELL  AND  CELL  DIVISION 


43 


part — the  cytoplasm,  nucleus,  the  centrosome,  and  even  many 
of  the  plastids.  So  that  the  final  result  of  the  process  is  typically 
the  formation  of  two  daughter  cells  similar  to  each  other  and 
also  to  the  parent  cell  in  all  essential  respects  save  in  size. 

The  division  of  cells  occurs  in  two  quite  dissimilar  ways.  The 
simpler  method,  and  the  less  frequent,  is  termed  direct  division 
or  amitosis  (Flemming).  Here  the  first  step  is  sometimes  the 
elongation  and  constriction  of  the  nucleolus,  when  this  is 
present,  into  two  separate  daughter  nucleoli,  or  in  other  cases 
the  appearance  of  a  new  second  nucleolus  (Fig.  19).  Next  the 
whole  nucleus  divides  into  two,  sometimes  by  simple  constric- 
tion into  two  separate  elements,  sometimes  by  the  ingrowth  of 


FIG.  19. — Amitosis  in  tendon  cells  of  a  new-born  mouse.     After  Nowikoff,  X  800. 

nc,    nucleolus. 

a  partition  wall,  or  in  still  other  cases,  by  the  formation  of  two 
new  nuclear  membranes  within  the  original  membrane,  the 
disappearance  of  the  latter  freeing  the  two  daughter  nuclei. 
This  division  of  a  nucleus  is  typically  followed  by  the  division 
of  the  cytoplasmic  portion  of  the  cell  which  is  ordinarily 
accomplished  by  the  development  of  a  cell  wall  between  the 
two  daughter  nuclei.  Very  frequently,  however,  division  of 
the  cell  body  does  not  follow  and  the  cell  remains  binucleate;  or 
this  process  of  nuclear  fission  may  be  repeated,  a  multinucleate 
cell  resulting,  such  as  a  striated  muscle  cell.  In  such  cases  of 
incomplete  cell  division  the  essence  of  the  process  seems  to  be 
the  rapid  increase  of  nuclear  surface  and  then  volume;  it  is 


44  GENERAL  EMBRYOLOGY 

usually  associated  with  special  forms  of  cell  activity.  Conditions 
in  some  of  the  Protozoa  suggest  that  primitively  division  of  the 
nucleus  or  the  multiplication  of  the  nuclear  bodies  might  not 
have  been  associated  with  a  corresponding  division  of  the 
cytoplasmic  body,  but  that  these  originally  independent 
divisions  have  gradually  come  to  be  uniformly  associated. 

It  has  commonly  been  supposed  that  the  direct  form  of  cell 
division  occurs  but  rarely  and  then  usually  in  cells  which  are 
moribund.  It  is  becoming  clear,  however,  that  amitosis  is  in 
reality  not  particularly  infrequent.  It  seems  to  occur  normally 
in  many  tissues  (mesenchyme),  and  often  where  there  is  an  un- 
usual or  a  sudden  increase  in  nuclear  activity  and  energy 
expenditure  on  the  part  of  the  whole  cell,  as  in  ovarian  follicle 
cells  and  tapetal  cells,  muscle  cells,  rapidly  growing  or  rediffer- 
entiating  cells  in  regenerating  tissues ;  it  is  also  true  that  amitosis 
is  frequent  in  such  tissues  as  stratified  epithelia  whose  cells 
are  nearing  the  end  of  their  life  or  activity. 

The  second  and  more  usual  method  of  cell  fission  is  that 
termed  indirect  cell  division,  or  mitosis  (Flemming),  or  karyo- 
kinesis  (Schleicher).  This  is  a  complicated  process  involving 
the  establishment  and  operation  of  an  intricate  mechanism 
within  the  cell,  shared  in  by  nearly  all  its  living  parts.  The 
essential  result  of  the  division  of  the  cell  by  the  action  of  this 
complex  mechanism  concerns  in  particular  the  chromatic  sub- 
stance of  the  nucleus,  for  in  nearly  all  known  instances  the 
chromatin  sharing  in  the  process  is  very  precisely  divided  into 
two  equal  portions,  each  of  which  goes  to  one  of  the  daughter 
cells.  We  may  give  here  only  a  brief  outline  of  the  essentials  of 
this  process  of  mitosis,  again  describing  an  imaginary  schema 
with  which  to  compare  later  some  of  the  variations  in  detail 
shown  by  actual  cells. 

As  the  first  step  in  mitosis  we  should  consider  the  division  of 
the  centrosome  into  two,  which  remain  lying  together  within 
the  undivided  kinoplasmic  centrosphere.  This  division  of  the 
centrosome  is  usually  quite  removed  in  point  of  time  from  the 
other  phenomena  of  mitosis,  for  it  occurs  normally  during  the 
reconstruction  of  a  daughter  cell  immediately  after  its  formation, 


THE  CELL  AND  CELL  DIVISION  45 

and  so  is  separated  by  a  considerable  intervening  vegetative 
period  from  the  other  events  of  mitosis  or  the  doubling  of 
other  parts  (Fig.  20,  A).  This  vegetative  phase  of  cell  life  is 
frequently  referred  to  as  the  "resting"  period  or  interkinesis; 
a  state  of  inaction  is  not  implied  by  the  term  "resting/'  for 
during  this  period  the  cell  is  performing  its  normal  and  char- 
acteristic functions  as  a  tissue  cell;  the  word  merely  indicates 
that  the  cell  is  not  undergoing  any  active  phase  of  division. 
The  termination  of  the  vegetative  phase  and  the  immediate 
inauguration  of  mitosis  is  ordinarily  first  distinguishable  in 
the  structure  of  the  nucleus.  The  chromatin  granules  become 
more  distinct,  enlarge  rapidly,  and  undergo  some  change  in 
chemical  constitution  indicated  by  an  increase  in  staining 
capacity  (Fig.  20,  A;  22,  A).  As  the  chromatin  increases  some 
of  the  granules  or  flakes  come  to  be  arranged  in  a  linear,  or 
sometimes  bilinear  series,  still  upon  some  of  the  linin  threads 
which  share  in  this  arrangement.  Thus  the  chromatin  and 
linin  form  a  tangled  thread  or  ribbon  called  the  skein  or  spireme 
(Figs.  20,  B;  21,  B;  22,  B). 

We  should  note  here  that  at  this  time  the  chromatin  of  the 
nucleus  which  is  not  included  in  the  spireme,  often  indeed  the 
greater  part  of  the  whole  amount  of  this  material,  is  thrown  out 
into  the  cytoplasm  and  dissolves  (Fig.  32);  the  more  fluid  parts 
of  the  nucleus  are  also  thrown  into  the  cytoplasm  by  the 
dissolution  of  the  nuclear  membrane.  It  may  thus  be  only  a 
comparatively  small  part  of  the  whole  nuclear  structure  that  is 
formed  into  the  spireme  proper. 

The  spireme  may  be  quite  continuous  throughout  the  nucleus, 
or  it  may  appear  from  the  first  as  a  fragmented  thread  com- 
posed of  several  short  pieces;  when  in  this  latter  condition  it  is 
spoken  of  as  a  segmented  spireme.  In  a  few  cases  the  spireme 
stage  is  largely  suppressed  and  the  chromatin  granules  collect 
immediately  into  compact  groups  without  indication  of  a 
skein  stage.  The  linin  network  in  part  becomes  a  sort  of  fine 
core  throughout  the  spireme  and  in  the  extra-chromatic  region 
remains  as  a  network  of  naked  fibers.  The  latter  portion  soon 
becomes  polarized  so  that  its  fibers  converge,  more  or  less 


46 


GENERAL  EMBRYOLOGY 


E 


FIG.  20. — Mitosis  in  cells  of  Salamandra  maculosa.  After  Prenant  and  Bouin. 
D,  H.  Primary  spermatocytes,  others,  spermatogonia.  A,  B,  C,  X  1000,  others, 
X  800.  A.  Inter  kinesis  or  resting  stage.  B.  Early  prophase;  spireme  continu- 


THE  CELL  AND  CELL  DIVISION 


47 


ous.  Centrosomes  omitted.  C.  Prophase;  spireme  segmented  into  chromo- 
somes. Centrosomes  commencing  to  diverge;  spindle  forming.  D.  Longi- 
tudinal splitting  of  chromosomes.  E.  Disappearance  of  nuclear  membrane; 
continued  divergence  of  Centrosomes  and  asters.  F.  Mesophase;  formation  of 
equatorial  plate.  Polar  view.  Chromosomes  V-shaped.  G.  Same  in  side  view. 
Only  a  few  of  the  chromosomes  are  shown.  H.  Anaphase;  daughter  chromo- 
somes diverging,  still  united  at  ends.  /.  Anaphase;  continued  divergence  of 
chromosomes,  now  entirely  separated.  J.  Late  anaphase;  complete  divergence 
of  chromosomes.  Spindle  breaking  down,  asters  disappearing.  K.  Telo- 
phase;  beginning  of  reconstruction  of  daughter  nuclei.  Chromosomes  disin- 
tegrating. L  Late  telophase;  division  completed.  Nuclei  reconstructed; 
centriole  divided;  cell  walls  completed.  Nuclear  membrane  forming,  c,  cen- 
trioles;  cs,  centrosphere;  n,  nucleus;  s,  spindle  remains;  sf,  spindle  fibers  cut  across. 


48 


GENERAL  EMBRYOLOGY 


FIG.  21. — Diagrams  of  the  process  of  mitosis.  From  Wilson,  "Cell,"  slightly 
modified.  A.  Resting-cell  with  reticular  nucleus  and  true  nucleolus;  at  c  the 
attraction-sphere  containing  two  centrosomes.  B.  Early  prophase;  the  chroma- 
tin  forming  a  continuous  spireme,  nucleolus  still  present;  above,  the  amphiaster 
(a).  C.D.  Two  different  types  of  later  prophases;  C.  Disappearance  of  the  pri- 
mary spindle,  divergence  of  the  centrosomes  to  opposite  poles  of  the  nucleus 
(examples,  many  plant-cells,  cleavage-stages  of  many  eggs).  D.  Persistence  of 
the  primary  spindle  (to  form  in  some  cases  the  "central  spindle"),  fading  of  the 


THE  CELL  AND  CELL  DIVISION  49 

distinctly,  toward  the  region  of  the  centrosphere;  in  many 
cells  such  a  polarization  of  the  linin  toward  the  centrosome 
exists  throughout  the  vegetative  phase.  The  two  centrosomes 
now  begin  to  diverge  and  the  surrounding  centrosphere  pulls 
in  two,  one  portion  accompanying  each  centrosome  (Figs.  20,  E; 
21,  D).  As  the  centrospheres  diverge  they  enlarge,  and  within 
each  appear  fibers  radiating  from  the  centrosome  as  a  center 
and  producing  the  asters.  ^Tiile  the  chromatic  and  achromatic 
parts  of  the  nucleus  have  been  passing  through  these  early 
stages  of  mitosis,  the  nucleolus  when  present  becomes  vacuo- 
lated,  commences  to  dissolve  and  finally  disappears.  Soon 
the  nuclear  membrane  also  commences  to  break  down  and 
dissolve,  first  in  the  region  of  the  asters,  leaving  the  nuclear 
substance  free  in  the  cytoplasm.  Next  the  chromatin  thread 
shortens  and  thickens,  breaking  in  the  case  of  the  continuous 
spireme,  into  a  number  of  separate  segments  or  rods;  or  if  the 
spireme  itself  is  of  the  segmented  type,  its  elements  now  shorten 
and  thicken.  When  the  spireme  segments,  the  linin  thread 
upon  which  the  chromatin  granules  are  strung  may  remain 
continuous  between  as  well  as  through  the  chromatic  rods. 
These  chromatic  segments  now  become  quite  homogeneous, 
clearly  differentiated  structures  called  the  chromosomes  (Figs. 
20  C,  E;  21,  D;  22,  C).  Strictly  speaking,  each  chromosome 
consists  of  a  dense  mass  of  fused  chromatin  granules  with  a 
portion  of  linin  embedded. 

In  practically  all  organisms  in  which  the  nucleus  is  a  definitely 
formed  structure,  the  number  of  chromosomes  appearing  during 
mitosis  is  fixed,  and  is  constant  throughout  all  divisions  of 

nuclear  membrane,  ingrowth  of  the  astral  rays,  segmentation  of  the  spireme- 
thread  to  form  the  chromosomes  (examples,  epidermal  cells  of  salamander,  forma- 
tion of  the  polar  bodies).  E.  Later  prophase  of  type  C;  fading  of  the  nuclear 
membrane  at  the  poles,  formation  of  a  new  spindle  inside  the  nucleus;  precocious 
splitting  of  the  chromosomes  (the  latter  not  characteristic  of  this  type  alone). 
F.  The  mitotic  figure  established.  G.  Metaphase;  splitting  of  the  chromo- 
somes (e.p.) ;  n,  the  cast-off  nucleolus.  H.  Four  stages  in  the  divergence  of  the 
two  halves  of  a  chromosome.  /.  Anaphase;  the  daughter-chromosomes  diverg- 
ing, between  them  the  interzonal  fibers  (i.f.),  or  central  spindle;  centrosomes  al- 
ready doubled  in  anticipation  of  the  ensuing  division.  J.  Late  anaphase  or 
telophase,  showing  division  of  the  cell-body,  mid-body  at  the  equator  of  the 
spindle  and  beginning  reconstruction  of  the  daughter-nuclei.  K.  Division 
completed. 


E 


FIG.  22. — Mitosis  in  the  segmenting  egg  of  the  clam,  Unio.  From  Dahlgren 
and  Kepner.  A.  Prophase  of  the  fourth  cleavage.  Chromatin  reticulum; 
centrosomes  on  opposite  sides  of  nucleus.  B.  Prophase.  Spireme  beginning  to 
segment  into  chromosomes;  nuclear  membrane  disappearing;  spindle  forming. 
C.  Late  prophase.  Chromosomes  formed;  spindle  becoming  completed;  nucle- 
olus  nearly  disappeared.  D.  Mesophase.  Chromosomes  in  equatorial  plate. 
E.  Early  anaphase.  Divergence  of  the  daughter  chromosome  groups.  F. 
Telophase.  Nuclear  division  completed  and  daughter  nuclei  reformed;  cyto- 
plasmic  division  commencing. 


THE  CELL  AND  CELL  DIVISION  51 

somatic  cells.  In  all  but  a  few  groups  the  chromosomes  appear 
in  an  even  number,  ordinarily  between  twelve  and  thirty-six, 
although  these  limits  are  frequently  passed.  The  chromosomes 
are  at  present  considered  the  most  important  elements  in  the 
cell,  and  interest  in  the  whole  process  of  mitosis  centers  in  their 
behavior.  The  details  of  the  process  of  mitosis  seem  directed 
toward  the  exactly  equal  division  and  distribution  of  these 
elements,  the  importance  of  which  justifies  the  more  detailed 
consideration  which  we  give  them  after  the  general  description 
of  mitosis  is  completed. 

While  the  chromosomes  are  forming,  the  centrosomes  and 
asters  continue  to  diverge,  passing  around  toward  opposite  sides 
of  the  nucleus.  The  linin  fibers  of  the  nucleus  tend  throughout 
to  remain  polarized  toward  the  centrosomes  and  the  separation 
of  these  bodies  from  one  another  draws  out  the  linin  fibers  into 
an  elongated  bundle  converging  at  each  end  toward  the  centro- 
some.  Finally  the  centrosomes  come  to  lie  on  exactly  opposite 
sides  of  the  nuclear  structures  and  as  the  nuclear  membrane 
disappears  completely  we  find  the  rays  of  the  asters  penetrating 
into  the  nuclear  region  and  forming,  together  with  the  linin, 
a  spindle-shaped  structure  lying  between  the  centrosomes,  its 
component  fibers  passing  among  the  chromosomes  (Figs.  20,  G; 
21,  F'j  22,  D).  In  many  cells  the  centrosomes  do  not  thus 
migrate  to  opposite  sides  of  the  nucleus,  but  separate  directly; 
the  nucleus  in  this  case  is  simply  drawn  up  to  lie  between  them 
(Fig.  21,  D).  The  result  is  the  same,  but  the  difference  in 
relative  behavior  of  the  centrosomes  and  nucleus  is  real  and 
must  be  taken  into  account  in  some  cases.  The  spindle  and 
asters  now  form  a  figure  resembling  a  diagram  of  the  lines  of 
force  within  a  simple  bipolar  magnetic  field.  This  figure  is 
called  the  amphiaster,  sometimes  the  achromatic  figure,  empha- 
sizing its  distinctness  from  the  chromatic  portion  of  the  nucleus, 
now  all  or  largely  included  in  the  chromosomes.  The  parts  of 
the  linin  network  directly  continuous  with  those  upon  which 
the  chromosomes  were  formed  originally,  seem  to  have  a  some- 
what different  history  from  the  remainder  of  the  linin.  They 
remain  attached  to  the  chromosomes  and  extend  thence  toward 


52  GENERAL  EMBRYOLOGY 

the  centrosomes,  forming  in  this  stage  a  superficial  sheath 
around  a  central  portion  of  the  spindle,  and  are  hence  termed 
the  mantle  fibers.  The  central  core  of  the  spindle  seems  in 
many  cases  to  be  formed  largely  from  the  remainder  of  the 
nuclear  linin,  though  in  other  cases  this  is  formed  in  the  same 
way  that  the  asters  are,  and  from  cytoplasmic  materials.  Thus 
the  amphiaster  is  usually  of  mixed  origin,  nuclear  and  cyto- 
plasmic, though  in  some  cases  the  spindle  at  least  seems  to  be 
wholly  nuclear  (linin);  the  asters  are  always  cytoplasmic  in 
origin.  All  these  fibers  form  definite  threads,  enlarged  as 
compared  with  the  original  linin  reticulum. 

The  definite  formation  of  the  chromatic  portion  of  the 
nucleus  into  chromosomes  and  the  achromatic  substance  into 
the  amphiaster  marks  the  termination  of  the  first  phase  of 
mitosis  which  is  known  as  the  prophase.  During  the  prophase 
there  has  occurred  the  actual  division  of  only  the  centrosome 
and  centrosphere;  the  other  important  changes  have  been 
preparatory  to  further  divisions — the  dissolution  of  the  nuclear 
membrane,  the  enlargement  and  rearrangement  of  the  chro- 
matin  granules,  the  formation  of  definite  chromosomes,  and  the 
establishment  of  the  achromatic  figure.  We  should  remember 
that  the  nucleolus  meanwhile  has  fragmented  and,  together 
with  a  large  or  small  amount  of  chromatin  which  is  not  formed 
into  chromosomes,  has  passed  out  into  the  cytoplasm  and 
disappeared. 

The  arrangement  of  the  materials  forming  the  achromatic 
figure  is  evidently  the  result  of  certain  tensions  within  the  cell, 
the  effect  of  which  is  first  to  draw  the  chromosomes,  until  now 
distributed  irregularly,  into  a  circle  about  the  equator  of  the 
spindle.  When  in  this  position  the  chromosomes  are  said  to 
form  the  equatorial  plate  (Figs.  20,  F,  G;21,F).  This  phase  of 
mitosis  is  also  in  general  preparatory  to  actual  division  but  it 
is  carried  on  after  the  division  mechanism  is  completely 
established.  This  period  of  division  is  known  as  the  mesophase 
(Lillie). 

Following  the  mesophase  is  the  metaphase.  The  chief  event 
of  this  phase  is  the  longitudinal  splitting  or  division  of  each 


THE  CELL  AND  CELL  DIVISION 


53 


chromosome  into  two  parallel  halves.  This  forms  two  equal 
and  similar  groups  of  daughter  chromosomes,  each  group 
similar  to  the  original  group  except  in  size.  As  a  matter  of 
fact  this  longitudinal  splitting  of  the  chromosomes  is  by  no 
means  always  deferred  until  this  time,  for  frequently  it  occurs 
during  the  pro  phase  of  division,  even  in  the  spireme  stage;  or 
rarely  the  chromatin  granules  may  divide  even  before  a  definite 
spireme  is  constituted  (Fig.  23) .  In  such  cases  the  chromosomes 
are  in  the  form  of  double  rods  throughout  the  entire  prophase 
and  mesophase;  the  metaphase  is  then  present  only  virtually. 

The  mantle  fibers  from  the  oppo- 
site poles  of  the  spindle  are  now 
attached  to  the  daughter  chromo- 
somes, usually  in  their  middles. 
Next  the  mantle  fibers  begin  to 
shorten  as  the  result  of  some  proc- 
ess centering  in  or  about  the  cen- 
trosomes  at  the  poles  of  the  spindle. 
As  the  poles  are  relatively  fixed, 
perhaps  by  the  anchoring  asters  or 
through  the  rigidity  of  the  central 
part  of  the  spindle,  the  result  is  the  FIG  23._Longitudinal  fission 

Separation  Of  the  tWO  groups  Of  of  the  spireme  in  the  division  of 
j  i,  -i  v  •  v  spore-mot  her-cell  in  Lilium  can- 

daugnter  Chromosomes  Which  move    didum.  After  Farmer  and  Moore. 

along  the  central  spindle  fibers  to- 
ward the  regions  of  the  centrosomes.  If  the  mantle  fibers 
are  attached  to  the  middle  of  the  chromosome  it  is  first 
drawn  out  into  a  3-  or  > -shape;  frequently  this  form  is 
assumed  during  the  mesophase,  the  apex  of  the  >  pointing 
centrally  in  the  equatorial  plate  (Figs.  20,  F,  H;  21).  The 
period  of  mitosis  occupied  by  the  divergence  of  the  two  chro- 
mosome groups  is  called  the  anaphase;  this  is  usually  very  brief 
as  the  chromosomes  diverge  rapidly.  Their  divergence  exposes 
the  central  fibers  of  the  spindle  which  are  then  called  the 
interzonal  or  connecting  fibers,  and  which  frequently  come  to 
have  an  important  share  in  the  formation  of  later  structures 
(Figs.  21,  H;  22,  E).  In  their  divergence  the  chromosomes 


54  GENERAL  EMBRYOLOGY 

themselves  seem  to  be  entirely  passive,  except  in  a  very  few 
isolated  instances  where  they  are  definitely  amoeboid  (Opalina) 
(Fig.  31).  The  two  chromosome  groups  are  finally  drawn 
completely  to  the  opposite  poles  of  the  spindle  and  the  process 
of  mitosis  then  enters  upon  its  final  period,  the  telophase. 

During  this  phase  the  cytoplasmic  portion  of  the  cell  becomes 
divided  into  two  parts,  usually  equal,  though  occasionally 
extremely  unequal.  Sometimes,  as  in  many  animal  cells,  this 
division  of  the  cytoplasm  results  from  its  peripheral  constriction 
in  a  plane  corresponding  with  that  of  the  equatorial  plate,  the 
constriction  deepening  until  the  cell  body  is  completely  severed 
and  the  two  daughter  cells  formed  (Figs.  20,  L;  21,  /,  J;  22,  F). 
More  frequently,  in  some  animal  and  in  practically  all  plant  cells, 
the  division  of  the  cytoplasm  results  from  the  formation  of  a 
partitioning  cell  wall,  in  the  formation  of  which  the  interzonal 
fibers  of  the  spindle  usually  take  an  important  part.  These  seem 
to  increase  in  number  and  to  thicken  in  the  middle,  ultimately 
fusing  and  forming  a  transverse  plate  which  is  the  rudiment  of 
the  future  cell  wall ;  the  remainder  of  the  wall  forms  as  a  distinct 
secretion  of  the  cytoplasm  in  that  region. 

As  the  diverging  chromosomes  approach  the  poles  of  the 
spindle  they  lose  their  distinct  outlines,  become  vesicular,  and 
gradually  lose  their  visible  identity  and  separateness  to  a  large 
extent;  with  a  few  exceptions  they  finally  seem  to  disintegrate 
completely  and  form  scattered  granules,  and  a  new  typical 
nucleus  is  constituted  in  each  daughter  cell.  Meanwhile  the 
centrospheres  and  asters  have  diminished  in  extent  and  clear- 
ness and  have  returned  again  to  the  condition  which  is  char- 
acteristic of  the  interkinesis.  About  the  new  nucleus  a  mem- 
brane is  formed,  either  from  the  nucleus  or  the  cytoplasm,  and 
the  mitosis  is  accomplished  (Figs.  20,  L;  21,  H-J).  In  most 
cases  just  about  this  time  the  centrosome  divides  in  prepa- 
ration for  the  next  mitosis.  During  the  interkinesis  the  nucleus 
and  cytoplasm  increase  in  size  and  soon  the  process  of  division 
is  repeated. 

The  length  of  time  occupied  by  the  whole  process  of  mitosis 
varies  greatly.  In  the  division  of  some  egg  cells  it  may  be 


THE  CELL  AND  CELL  DIVISION  55 

completed  in  fifteen  minutes,  or  it  may  occupy  one  or  even  two 
hours,  and  in  some  special  cases  a  much  longer  period. 

This  account  of  mitosis,  although  brief  and  including  only 
some  of  the  essentials,  brings  out  clearly  the  unity  of  the 
nucleus  as  an  organ;  it  behaves  as  a  more  or  less  separate 
unit  of  cell  organization  throughout  all  this  intricate  process. 
And  we  see  clearly  this  extremely  important  fact,  that  the 
nucleus  of  a  cell  is  formed  from  a  preexisting  nucleus  of 
the  same  constitution.  Nuclei  arise  only  from  preexisting 
nuclei;  there  is  a  nuclear  continuity  quite  parallel  with  cell 
continuity.  And  going  one  step  farther,  it  is  probable  that 
chromosomes  are  derived  only  from  preexisting  chromosomes. 
This  idea  of  genetic  continuity  is  not  completely  applic- 
able to  all  cell  organs,  however,  for  occasionally  the  centrosomes 
are  not  derived  from  preexisting  centrosomes,  but  may  arise 
de  novo,  and  in  the  development  of  the  new  organism  the 
centrosome  is  typically  derived  from  the  sperm  cell  alone. 
Among  the  plants  many  of  the  plastids  seem  to  be  genetically 
related  and  to  be  formed  by  the  division  of  preexisting  plastids. 
The  other  less  living  cell  structures  are  usually  distributed 
passively  to  the  daughter  cells,  and  such  structures  may  be 
formed  anew  in  the  new  cells. 

The  relation  between  the  direction  of  the  plane  of  cell  division 
and  the  general  morphology  of  the  cell  body  demands  a  word. 
From  the  preceding  account  it  is  obvious  that  the  plane  of 
division  is  at  right  angles  to  the  longitudinal  axis  of  the  spindle, 
but  the  position  which  the  spindle  assumes  is  itself  prede- 
termined. The  position  of  the  spindle  axis  is  fixed  by  the 
location  of  the  centrosomes.  When  the  single  centrosome 
divides  and  the  daughter  centrosomes  pass  to  opposite  sides  of 
the  nucleus,  they  usually  migrate  equal  distances  from  the  ori- 
ginal position  of  the  centrosome;  it  follows,  therefore,  that  this 
region  falls  within  the  plane  of  the  equator  of  the  spindle  and 
consequently  in  the  plane  of  the  new  division.  When  the 
centrosome  does  not  alter  materially  its  relative  position  in  the 
cell,  the  next  division,  again  being  through  the  plane  occupied 
by  the  centrosome  and  the  center  of  the  nucleus,  will  be  hi 


56 


GENERAL  EMBRYOLOGY 


general  at  right  angles  to  the  preceding  division  (Fig.  24).  Thus 
the  planes  of  succeeding  divisions  tend  to  alternate,  each  per- 
pendicular to  the  preceding.  Any  other  relation  involves  the 
migration  of  the  centrosome  from  the  position  originally 
occupied  by  it  in  the  daughter  cell;  or  the  spindle  may  change 
its  position  during  or  after  its  formation,  and  this  regular 
relation  thus  be  disturbed.  Typically  the  spindle  takes  such 
a  position  that  its  long  axis  lies  in  the  direction  of  the  greatest 
protoplasmic  extent  of  the  cell  (0.  Hertwig),  a  position  which 
would  result  from  the  tensions  between  a  comparatively 
elongated  body  and  a  fluid  medium  in  which  it  is  suspended 


III 


FIG.  24. — Diagram  illustrating  the  relation  between  the  position  of  the  centro- 
some and  the  plane  of  cell  division.  Symmetrical  motion  of  the  daughter  centro- 
somes  results  in  the  regular  alternation,  at  right  angles,  of  successive  division 
planes. 

and  free  to  move  in  any  direction.  There  are  many  ex- 
ceptions to  these  two  general  rules  in  special  conditions,  such 
as  simple  columnar  epithelia,  stratified  epithelia,  the  maturing 
germ  cells,  etc.;  these  indicate  perhaps  that  a  more  funda- 
mental cause  of  the  direction  of  cell  division  remains  to  be 
discovered. 

Some  important  modifications  of  this  schema  of  cell  division  given 
above  will  be  noted  in  other  connections,  but  a  few  special  conditions 
are  conveniently  mentioned  here.  The  most  important  and  fundamental 
modifications  are  doubtless  those  which  occur  during  the  forming  and 
maturing  of  the  germ  cells ;  these  are  to  be  described  in  detail  in  Chapter 
IV,  and  here  we  should  only  note  that  the  behavior  of  the  chromosomes 


THE  CELL  AND  CELL  DIVISION 


57 


in  these  divisions  is  very  complex;  that  there  is  here  only  one-half  the 
number  of  these  bodies  formed  in  the  cells  of  the  somatic  tissues;  that 
mitoses  may  occur  without  an  intervening  interkinesis ;  that  in  the 
case  of  the  egg  cell  the  division  may  be  of  extreme  inequality,  so  that 
one  of  the  daughter  cells  is  like  an  extremely  small  bud,  although  with 
respect  to  nuclear  structure  the  two  cells  are  alike ;  and  that  in  certain 
special  mitoses  one  or  more  of  the  chromosomes  may  fail  to  divide  and 
therefore  may  be  unrepresented  in  one  of  the  daughter  cells. 

Among  the  higher  plants  an  important  characteristic  is  the  absence 
of  definite  centrosomes  and  asters,  although  these  structures  are 
normally  present  among  the  lower  plants.  The  absence  of  centrosomes 
results  in  the  formation  of  a  characteristically  blunt  or  truncated 


FIG.  25. — A.  Multipolar  spindle  in  spore-mother-cell  of  Equisetum.  From 
Wilson,  "Cell,"  after  Osterhout.  B.  Intranuclear  spindle  in  the  oocyte  of  the 
Copepod,  Canthocamptus  staphylinus.  From  Hegner.  X  850. 

spindle  in  most  of  the  higher  plants.  In  some  animals  the  spindle  is 
rather  truncated  also,  but  this  is  usually  found  to  be  in  reality  multi- 
polar,  composed  of  many  small  bundles  of  spindle  fibers  terminating 
in  a  row  of  centrosomes  or  centrosome-like  bodies  (Fig.  25,  A).  In  the 
tissue  cells  of  most  animals  the  asters  are  relatively  small,  though  the 
spindle  remains  large  and  distinct;  in  a  few  cases  it  seems  that  even  in 
animal  cells  division  may  be  effected  in  the  absence  of  centrosomes. 

Many  significant  modifications  of  mitosis  occur  among  the  Protozoa, 
where  we  find  certain  conditions  which  seem  to  offer  suggestions  as  to 
the  evolution  of  the  mitotic  figure  and  process,  as  well  as  of  some  of  the 
chief  cell  organs  themselves.  Among  these  forms  the  process  of  divi- 
sion is  often  complicated  through  its  being  at  the  same  time  the  essential 
step  in  reproduction,  rather  than  merely  a  step  in  or  condition  of  differ- 
entiation, as  in  the  Metazoan.  Thus  the  process  of  budding  or  gemma- 
tion is  essentially  an  unequal  cell  division;  and  in  brood  ("spore")  for- 
mation we  see  a  form  of  cell  division  in  which  the  nucleus  divides  many 


58 


GENERAL  EMBRYOLOGY 


times  without  any  corresponding  cytoplasmic  divisions,  until  finally,  all 
at  once,  the  cytoplasm  is  cut  into  a  large  number  of  small  cells.  In 
these  forms  of  division  no  mitotic  figure  is  formed  ordinarily,  and  one 
of  the  modifications  due  to  association  with  reproduction  is  seen  in  the 
fact  that  the  resulting  bud,  or  brood-cell,  may  have  a  form  and  structure 
entirely  unlike  that  of  the  mother  cell.  In  budding,  especially  in  that 
form  called  bud-fission,  the  nucleus  does  not  ordinarily  divide  until 


A  B 

FIG.  26. — Nuclear  division  in  the  Ciliate,  Dileptus,  From  Calkins,  "Proto- 
zoology." A.  Vegetative  form.  Nucleus  in  the  form  of  chromatin  granules 
scattered  through  the  greater  part  of  the  cell  ("distributed  nucleus").  B. 
During  division.  Each  chromatin  granule  elongates  and  divides  into  two. 

after  the  bud  is  practically  completed  and  ready  to  be  cut  off,  i.e., 
cytoplasmic  division  tends  to  precede  nuclear  division.  Another  com- 
plication due  to  the  same  association  is  the  frequent  differentiation  of 
two  forms  of  chromatic  substance  in  the  nucleus.  These  are  the  repro- 
ductive chromatin,  or  idiochromatin,  and  the  vegetative,  or  tropho- 
chromatin  (Dobell) ;  in  some  Protozoa  these  may  come  to  be  organized 
into  two  separate  nuclei  which  are  sometimes  equivalent  to  what  are 
called  the  micro-  and  macronucleus  respectively.  We  have  already 
mentioned  the  distributed  nucleus  of  many  unicellular  organisms  in 
which  the  chromatin  is  not  organized  into  a  definite  nuclear  organ, 
but  is  in  the  form  of  scattered  granules  or  collections  of  granules 


THE  CELL  AND  CELL  DIVISION 


59 


throughout  the  cell  (Fig.  26,  A).  Such  a  condition  indicates  strongly 
that  the  nuclear  and  cytoplasmic  parts  of  the  cell  have  arisen  through 
the  gradual  differentiation  of  a  common  protoplasmic  basis.  In  cell 
division  each  of  these  chromatic  bodies  may  first  divide  into  two 
(Dileptus),  though  the  members  of  the  resulting  pair  are  not  distributed 
to  different  daughter  cells,  for  the  accompanying  division  of  the  cell 
is  completed  without  any  rearrangement  of  the  chromatin  granules 
(Fig.  26,  B).  In  other  forms  with  distributed  nuclei  (Tetramitus),  the 
scattered  granules  collect  about  an  active  kinoplasmic  organ  termed 
the  division  center  (Fig.  27) ;  this  divides,  and  the  two  products  separate, 


FIG.  27. — Cell  division  in  the  Flagellate,  Tetramitus.  After  Calkins.  A 
Vegetative  condition  showing  scattered  chromatin  granules  (distributed  nucleus) 
and  division  center.  B.  Collection  of  chromatin  granules  preparatory  to  divi- 
sion. C.  Fission  of  the  division  center.  D.  Separation  of  the  division  centers 
accompanied  by  the  daughter  groups  of  granules  (nuclei). 

each  accompanied  by  a  group  of  chromatic  granules  which  are  then 
redistributed  equally  to  the  daughter  cells,  although  no  definite  mitotic 
figure  is  formed.  Even  when  a  definite  nucleus  does  come  to  be  estab- 
lished, much  of  the  chromatin  of  the  cell  may  not  be  contained  within 
it  but  may  remain  distributed  (Heliozoa,  Radiolaria) .  Finally,  of  course, 
all  of  the  chromatin  becomes  contained  within  one  or  more  definite 
nuclear  structures  which  may  be  simple  spherical  bodies,  or  they  may 
show  considerable  complication  and  variation  in  form.  A  definite 
nuclear  membrane  may  be  absent  at  first,  as  in  Chilomonas  and 
Trachelomonas,  though  it  is  formed  in  practically  all  cases  where  the 
chromatin  granules  form  definite  nuclear  groups.  Within  the  nucleus 
the  chromatic  substance  may  not  be  definitely  organized  into  chromo- 
somes, or  these  bodies  may  appear  only  in  certain  divisions  associated 
with  gametic  reproduction  - (Paramcecium) ;  in  some  Protozoa,  however, 
definite  chromosomes  are  typically  established  and  become  clearly 
marked  during  each  mitosis  (Fig.  28). 


60 


GENERAL  EMBRYOLOGY 


Probably  the  most  interesting  modifications  of  mitosis  among  the 
Protozoa  are  those  connected  with  the  formation  and  behavior  of  the 
centrosome  and  mitotic  spindle,  upon  the  origin  of  which  they  may 


FIG.  28. — Nuclear  division  in  the  Rhizopod,  Euglypha.  After  Schewiakoff. 
X  800.  A.  Coarse  network  of  chromatin.  B.  Contraction  of  chromatin  fibers 
and  beginning  of  formation  of  loops  (chromosomes).  C.  Arrangement  of  chro- 
mosomes in  equatorial  plate.  Polar  view.  D.  Equatorial  plate.  Lateral  view. 
Spindle  forming.  E.  Splitting  of  chromosomes  and  beginning  of  divergence. 
F.  Continued  divergence  of  chromosomes.  G.  Division  nearly  completed. 

perhaps  throw  some  light.  In  some  species  there  is  no  indication  of 
a  specialized  organ  concerned  particularly  with  division.  In  a  few 
forms,  as  mentioned  above,  a  division  center  may  be  formed,  although 


FIG.  29. — Nuclear  division  in  the  Rhizopod,  Centropyxis  aculeata.  After 
Schaudinn  (Doflein).  A.  Nucleus  in  vegetative  stage.  B.  Appearance  of 
centriole.  C.  Equatorial  plate.  Spindle  with  centrosomes;  plasma  radiations, 
D.  Beginning  of  reconstruction  of  daughter  nuclei. 

no  definite  nucleus  is  present,  the  distributed  chromatin  granules 
collecting  into  a  single  group  only  at  the  time  of  cell  fission.  In  some 
species  of  Amoeba,  and  in  a  few  other  forms,  one  of  the  large  chromatin 


THE  CELL  AND  CELL  DIVISION 


61 


bodies,  or  karyosomes,  within  the  nucleus  is  specialized  as  an  organ  of 
division,  called  the  central  body  and  functionally  equivalent  to  a  centro- 
some  (Fig.  29).  This  body  does  not  lose  its  chromatic  character,  may 
be  surrounded  by  a  definite  membrane,  and  appears  to  have  functions 
other  than  those  of  a  centrosome  which  it  exercises  during  the  intervals 
between  divisions.  In  cell  division  this  central  body  remains  wholly 
or  in  part  intranuclear.  Apparently  this  represents  a  very  early  stage 


FIG.  30. — Nuclear  division  in  the  Rhizopod,  Chlamydophrys  stercorea.     After 
Schaudinn  (Doflein).     A.  Nucleus  with  central  body  and  chromatin  threads. 

B.  Elongation  of  central  body  and  beginning  of  formation  of  equatorial  plate. 

C.  Equatorial  plate.     Central  body  spindle-shaped  with  polar  centrosome-like 
thickenings.     D.  Equatorial  plate  divided  into  two.     Plasma  radiations  from 
the   "centrosomes."     E.   Fission   of   central   body   and   chromatin   masses.     F. 
Division  completed.     Daughter  nuclei  being  reconstructed. 


in  the  differentiation  from  a  chromatic  body  of  the  centrosome  which 
later  becomes  wholly  achromatic  and  typically  extranuclear.  Several 
other  forms  have  a  dynamic  division  center  equivalent  to  the  centrosome 
but  intranuclear;  a  nucleus  of  this  type  is  known  as  a  centronucleus  (Fig. 
30;  see  also  Fig.  25,  B) .  In  the  division  of  such  nuclei  the  nuclear  mem- 
brane may  remain  entirely  intact  (Euglena)  or,  as  in  Noctiluca,  the 
nuclear  membrane  may  partly  break  down  during  mitosis.  In  several 
forms  possessing  a  definite  extranuclear  centrosome  this  body  remains 
undivided  in  cell  fission,  passing  to  one  daughter  cell  alone,  while  a 
new  centrosome  develops  in  the  other  cell;  but  this  forms  first  as  an 


62 


GENERAL  EMBRYOLOGY 


intranuclear  structure  which  later  moves  out  into  the  cytoplasm  along- 
side the  nucleus.  These  conditions  indicate  strongly  the  nuclear  origin 
of  the  centrosome.  And  there  is  some  reason  for  believing  the  spindle 
also  originally  a  nuclear  structure,  as  it  still  is,  in  part  at  least.  The 
spindle  is  a  less  constant  organ  than  the  centrosome,  compared  with 
which  it  is  of  secondary  importance.  In  several  Protozoa  and  simple 
plants  the  spindle  is  entirely  absent,  usually  where  the  centrosome  is 
intranuclear,  so  that  no  definite  mitotic  figure  is  formed.  In  other  forms 
the  spindle  is  intranuclear,  and  then  the  centrosomes  or  their  equivalents 
may  be  absent,  as  in  Opalina  (Fig.  31).  This  form  is  further  remark- 
able for  the  chromatic  character  of  some  of  its  spindle  fibers,  and  in 


FIG.  31. — Nuclear  division  in  the  Infusorian,  Opalina  intestinalis.  After 
Metcalf.  X  1200.  A.  Nucleus  in  anaphase  showing  chromatic  reticulum, 
which  may  be  regarded  as  equivalent  to  the  spindle,  and  branched,  amoeboid 
chromosomes.  B.  Late  telophase. 


that,  in  the  absence  of  centrosomes,  the  chromosomes  separate  by  an 
active  amoeboid  movement,  suggesting  a  possibly  primitive  behavior 
of  the  chromosomes  in  cell  division  (Metcalf) . 

It  is  of  course  impossible  now  to  get  definite  information  regarding 
the  evolution  of  the  cell  organs  and  the  process  of  mitosis,  and  in  these 
connections  the  conditions  found  at  present  among  the  Protozoa  are 
only  suggestive.  Many  of  these  conditions  do  suggest  strongly,  how- 
ever, that  the  nucleus  has  been  developed  from  the  gradual  aggregation 
of  scattered,  chemically  differentiated  particles ;  that  within  the  nucleus 
certain  chromatic  elements  became  specialized  into  division  centers 
which  finally  became  extranuclear,  achromatic  bodies — the  centro- 


THE  CELL  AND  CELL  DIVISION  63 

somes ;  and  that  certain  achromatic  elements  became  specialized  into  a 
spindle,  also  at  first  intranuclear  (linin),  and  at  present  in  nearly  all 
forms  both  intra-  and  extranuclear  in  origin.  The  chromosomes  we 
must  now  consider  more  particularly. 

Interest  in  the  process  of  mitosis  centers  in  the  chromosomes 
and  their  behavior,  for,  as  we  have  said,  this  whole  process  seems 
to  be  directed  toward  the  equal  distribution  of  the  daughter 
chromosomes  to  the  daughter  cells,  while  the  cytoplasm  may 
or  may  not  be  equally  divided  at  the  same  time.  We  do  not 
know  how  the  constituents  of  the  nucleus  other  than  the 
chromosomes  are  distributed  in  cell  division.  There  is  little 
reason  for  supposing  that  these  are  distributed  with  exact 
equality. 

We  may  recognize  two  chief  aspects  of  chromosomal  behavior 
in  mitosis.  The  first  is  the  division  of  the  chromatin  granules 
or  chromioles  composing  the  chromosomes;  these  granules  all 
divide  in  the  same  direction  so  that  the  total  result  appears  as 
an  exactly  equal  longitudinal  division  of  each  chromosome,  or 
of  the  spireme  in  those  cases  where  the  division  of  the  granules 
occurs  very  early.  This  is  the  essential  act  of  chromosome 
reproduction  and  it  is  obviously  a  process  concerning  the 
chromatin  alone,  independent  of  the  remainder  of  the  mitotic 
mechanism.  The  second  important  fact  is  the  distribution  of 
the  two  chromosome  halves  or  daughter  chromosomes  to  the 
two  daughter  cells.  This  is  accomplished  by  the  extra-chromo- 
somal elements  of  the  mitotic  figure,  the  chromosomes  apparently 
taking  a  purely  passive  part  in  the  process.  We  see  in  the 
mitotic  figure,  not  a  mechanism  for  cell  division  merely,  for  this 
is  frequently  accomplished  in  the  absence  of  mitosis,  nor  for 
chromosome  division,  for  this  frequently  precedes  the  formation 
of  the  mitotic  figure;  but  essentially  the  mitotic  figure  is  a 
mechanism  effecting  the  equal  distribution  to  the  daughter 
cells  of  the  products  of  chromosomal  division  within  the  nucleus 
of  the  parent  cell,  so  that  each  new  cell  has  a  complete  group  of 
chromosomes  similar  to  those  of  the  parent  cell.  The  precision 
and  wide  occurrence  of  this  equal  distribution,  through  mitosis, 
of  the  chromosomes  and  of  these  cell  organs  alone,  leads  to 


64  GENERAL  EMBRYOLOGY 

the  assumption  that  the  chromosomes  constitute  the  essential 
physiological  elements  of  the  nucleus  and  therefore  of  the  cell. 
There  are  many  subsidiary  facts  indicating  the  great  importance 
of  these  bodies.  Consideration  of  the  relation  of  the  chromo- 
somes to  the  special  problems  of  heredity  and  sex  will  be  deferred 
to  a  later  chapter  (Chapter  VII)  for  fuller  consideration,  but 
we  should  mention  here  a  few  of  the  important  facts  and  hypo- 
theses regarding  these  bodies. 

In  the  nuclei  of  many  of  the  Protozoa  definite  chromosomes 
are  already  present,  but  in  some  unicellular  organsims  there  are 
conditions  suggesting  a  possible  mode  of  evolution  of  these 
structures.  We  have  mentioned  the  collection  of  the  distrib- 
uted chromatin  granules  into  small  groups  through  the  cell; 
these  groups  have  been  regarded  as  the  rudiments  of  chromo- 
somes. After  a  definite  nucleus  is  established  the  chromatin 
granules  remain  as  definite  bodies,  and  each  divides  in  cell 
fission.  Even  when  the  chromatin  granules  merely  become 
rearranged  about  a  division  center,  as  in  Chilomonas  and 
Trachelomonas,  although  definite  chromosomes  may  not  be 
formed,  the  division  of  the  granules  occurs  as  the  essential  step 
in  fission,  just  as  later  when  the  granules  collect  and  fuse  into 
chromosomes.  In  Paramcecium  definite,  chromosomes  are 
formed  only  in  certain  divisions,  namely,  those  immediately 
preceding  conjugation,  that  is  during  garnet ogenesis  (Calkins 
and  Cull,  Fig.  82),  and  it  is  but  a  short  step  from  this  condition 
to  the  regular  formation  of  chromosomes  in  all  mitoses. 

In  the  reconstruction  of  the  daughter  nucleus  the  chromo- 
somes become  vacuolated  and  finally  break  up  into  scattered 
granules  whose  distribution  through  the  nucleus  is  so  irregular 
that  in  nearly  all  cases  no  trace  of  chromosomal  structure  is 
apparent.  The  nucleus  then  grows  rapidly,  the  chromatin 
content  often  increasing  to  many  times  that  in  the  original 
chromosome  group,  until  it  soon  reaches  a  quite  definite  size, 
varying  widely  in  different  cells,  but  fairly  constant  for  cells  of 
a  single  kind  in  a  given  species.  The  nucleus  and  chromatin 
then  remain  without  much  further  change,  quantitative  at 
any  rate,  until  toward  the  close  of  the  vegetative  or  normally 


THE  CELL  AND  CELL  DIVISION 


65 


functional  period  of  cell  life.  At  the  time  of  the  next  division 
much  of  the  chromatin  is  usually  eliminated  from  the  nucleus, 
is  cast  out  into  the  cytoplasm  and  disappears  along  with  the 
nucleolus  (Fig.  32);  the  chromosomes  which  then  appear, 


FIG.  32. — Early  cleavages  of  the  egg  of  the  Nematode,  Ascaris.  Origin  of 
primordial  germ  cells  and  casting  out  of  chromatin  in  the  somatic  cells.  From 
Wilson,  "Cell."  after  Boveri.  A.  Two-cell  stage  dividing;  polar  view,  s,  stem- 
cell,  from  which  the  germ  cells  are  derived.  B.  Later  stage  of  same  division; 
lateral  view,  c,  chromatin  in  somatic  cell  being  thrown  out  into  the  cytoplasm. 
C.  Completion  of  four-cell  stage  showing  eliminated  chromatin.  D.  Division  of 
four-cell  stage  showing  continued  chromatin  elimination  in  the  third  somatic 
cell. 

or  reappear,  are  similar  in  every  respect  to  those  in  the  preceding 
division  (Van  Beneden).  The  chromosomes,  therefore,  show 
the  same  specific  constancy  as  any  other  characteristic  of  the 
organism. 


66  GENERAL  EMBRYOLOGY 

The  most  obvious  characteristic  of  the  chromosomes  is  that 
of  numerical  constancy.  In  different  species  of  organisms  the 
number  varies  greatly  but  there  is  in  general  little  if  any 
relation  between  the  grade  or  relative  complexity  of  the 
organism  and  the  number  of  chromosomes  in  its  nuclei;  closely 
related  species  of  a  single  genus  may  differ  widely,  e.g.,  Ascaris 
megalocephala  has  four,  A.  lumbricoides  forty-eight.  In  general 
the  number  seems  highest  in  some  of  the  Protozoa.  Where 
there  are  very  many  minute  chromosomes  the  difficulty  of 
counting  them  exactly  is  very  great  and  it  cannot  then  be 
said  precisely  what  or  how  constant  the  number  is.  In 
Mastigella  there  are  about  forty,  in  Actinosphcerium  one  hundred 
and  thirty  to  one  hundred  and  fifty,  in  Paramcecium  about  two 
hundred.  Among  the  Metazoa  the  smallest  number  is  two,  in  a 
variety  of  Ascaris,  the  largest  known  is  one  hundred  and  sixty- 
eight,  in  Artemia.  Frequent  numbers  are  twelve,  sixteen,  and 
twenty-four,  but  any  number  may  be  found  within  these  known 
limits.  The  number  is  practically  always  an  even  one  in 
somatic  cells,  even  or  odd  in  the  germ  cells  or  their  immediate 
predecessors.  The  numbers  found  in  the  tissue  cells  of  some 
of  the  familiar  organisms  are  the  following:  rat,  guinea-pig, 
ox,  sixteen;  Amphioxus,  salmon,  salamander,  frog,  mouse, 
man  (female),  twenty-four;  earthworm,  thirty-two;  shark, 
thirty-six;  sea-urchin,  eighteen  in  one  species,  thirty-six  or 
thirty-eight,  in  another;  pine,  onion,  sixteen;  lily,  peony, 
twenty-four. 

While  the  number  of  chromosomes  is  thus  practically  constant 
it  is  not  absolutely  invariable  and  deviations  from  the  normal 
are  now  known  to  occur  in  several  forms.  Of  course  the  most 
frequent  variation  is  the  typical  reduction  of  the  number  to 


-j,   during  certain  phases  in  the  for- 

mation of  the  germ  cells  (Van  Beneden),  or  throughout  the 
gametophytic  generation  of  many,  perhaps  most,  plants. 
But  as  we  shall  see  later,  this  should  hardly  be  called  a  variation 
from  normal.  A  deviation  of  an  entirely  different  kind  is  seen 

o 

in  a  few  cases  where  the  chromosome  number  is  =   in  cleavage 


THE  CELL  AND  CELL  DIVISION  67 

or  tissue  cells  of  certain  individuals;  thus  in  the  cleavage  cells  of 
Ascaris  megalocephala  the  number  is  two  or  four,  in  the  tissues  of 
Helix  pomatia,  twenty-four  or  forty-eight,  in  StrongylocentrotuSj 
eighteen  or  thirty-six.  In  such  cases  each  of  the  lesser  number 
is  said  to  be  bivalent,  and  it  is  supposed,  not  without  reason, 
that  each  is  actually  composed  of  two  ordinary  or  univalent 
chromosomes.  In  a  few  instances  the  number  may  be  even 
less  than  one-half  the  normal  and  each  is  then  said  to  be  pluri- 
r alent;  thus  in  the  formation  of  the  embryo-sac  in  the  lily  a 
variation  in  certain  nuclei  has  been  found,  the  number  varying 
by  fours  from  twelve  to  twenty-four  (s  =  24).  The  significance 
of  these  unusual  cases  is  varied  and  sometimes  doubtful. 
Again,  constant  differences  in  chromosome  number,  in  both 
somatic  and  germ  cells,  are  associated  with  sex  differences  in  a 
large  number  of  species  of  several  widely  divergent  phyla.  In 
such  cases  the  females  have  a  somatic  group  from  one  to  five, 
or  even  more,  in  excess  of  that  of  the  male;  in  such  cases  the 
specific  number  is  fixed,  though  some  variable  species  are 
known. 

None  of  these  unusual  deviations  from  the  normal  is  of  the 
character  of  a  "  normal  variability."  Indeed  very  few  in- 
stances of  this  kind  of  variability  in  chromosome  number  are 
known.  One  instance  is  that  of  the  salamander  larva,  in 
certain  tissue  cells  of  which  the  number  is  said  to  vary  (Delia 
Valle)  in  different  individuals  from  nineteen  to  forty,  the  normal 
being  twenty-four,  and  in  a  single  specimen  limits  of  nineteen 
and  twenty-seven  have  been  described. 

This  very  high  degree  of  specific  numerical  constancy  of  the 
chromosomes  indicates  strongly  that  the  appearance  of  a 
chromosome  in  mitosis  is  not  determined  by  a  large  number  of 
causes,  but  that  it  is  the  result  of  the  operation  of  a  single  and 
simple  factor;  what  this  may  be  is  a  matter  of  conjecture  only. 

Another  important  characteristic  of  the  specific  chromosome 
group  is  the  constancy  of  the  form  and  size  differentiations 
among  the  members  of  the  group.  In  many  organisms  it  can 
be  seen  clearly  that  the  chromosomes  of  a  single  nucleus  are 
not  all  of  the  same  size  and  form;  they  may  differ  in  shape, 


68 


GENERAL  EMBRYOLOGY 


dimensions,  and  proportions  (Fig.  33)  (Montgomery).  More- 
over these  differences  are  constant  from  one  cell  generation  to 
the  next,  so  that  similar  chromosomes  may  be  identified  in 
successive  mitoses.  The  form  is  somewhat  more  variable  than 
the  volume,  which  is  remarkably  uniform.  It  should  be  said 
that  the  size  of  a  given  chromosome  is  not  fixed  throughout  the 
entire  cell  history,  for  at  certain  periods,  particularly  in  the 


FIG.  33. — Various  chromosome  groups  illustrating  variation  in  size  and  form, 
and  coupling  of  chromosomes.  A,B,  from  Sutton,  C,  after  Wilson,  D,  after  Agar. 
A,B.  Spermatogonia  of  the  grasshopper,  Brachystola  magna.  C.  Spermato- 
gonium  of  the  squash-bug,  Anasa  tristis.  D.  Spermatogonium  of  the  lung-fish, 
Lepidosiren. 

germinal  tissues,  the  chromosomes  may  be  many  times  larger 
than  at  other  periods  (Fig.  34).  But  at  corresponding  cell  ages 
the  corresponding  chromosomes  are  practically  equal  in  volume, 
and  in  somatic  cells  such  volume  changes  of  single  chromosomes 
are  relatively  infrequent. 

One  most  significant  and  very  important  fact  in  this  con- 
nection is  that  in  the  somatic  cell  the  chromosomes  are  pres- 
ent in  couples  of  similar  elements;  there  are  two  of  each  size  or 
form  (Montgomery)  (Figs.  33,  72,  A;  142,  A).  The  exceptions 


THE  CELL  AND  CELL  DIVISION 


69 


are  found  chiefly  in  those  forms  where  sex  differences  are  found ; 
in  such  cases  one  or  more  chromosomes  are  unpaired,  or  the 
members  of  the  pair  may  be  dissimilar  in  size.  And  further 

D 

in  the  germ  cells  with  —  chromosomes,  none  is  paired — all  are 

single,  each  somatic  pair  is  represented,  and  the  groups  in  eggs 
and  sperm  are  alike. 


FIG.  34. — Changes  in  the  volume  of  chromosomes  in  the  egg  of  the  Elasmo- 
branch,  Pristiurus.  All  drawn  to  same  scale.  From  Wilson,  "Cell,"  after 
Riickert.  A.  In  egg  of  3  mm.  diameter.  Chromosomes  at  maximal  size  and 
minimal  staining  capacity.  B.  In  egg  of  13  mm.  diameter.  C.  In  fully  grown 
ovarian  egg.  Minimal  size  and  maximal  staining  capacity. 

Such  facts  as  those  given  above  taken  in  connection  with  the 
precision  with  which  each  chromosome  is  halved  in  mitosis,  lead 
almost  irresistibly  to  the  supposition  that  the  chromosomes 
must  be  qualitatively  unlike.  Such  qualitative  differences 
cannot  be  observed  directly,  and  can  only  be  inferred,  but  as 
we  shall  see  in  connection  with  the  relation  of  the  chromosomes 
to  heredity,  this  inference  seems  to  be  justified  from  the  results 
of  the  experimental  or  accidental  modification  of  the  chromo- 
somal content  of  the  nuclei  and  the  character  of  the  resulting 
cells  or  cell  groups  (Chapter  VII). 


70  GENERAL  EMBRYOLOGY 

In  connection  with  the  chromosomes  there  remain  to  be  mentioned 
two  important  hypotheses.  The  first  is  the  hypothesis  of  the  speci- 
ficity of  the  chromosomes.  Stated  in  its  barest  form  the  essence  of 
this  idea  is  that  each  chromosome  functions,  in  cell  life,  in  its  own 
particular  way,  representing  a  center  for  reactions  of  a  specific  kind 
only;  that  the  chromosomes  are  cell  "organs,"  functionally  differentiated 
and  representing  a  division  of  labor  roughly  analogous  to  the  functional 
differentiations  of  the  whole  animal  body.  It  is  impossible  to  discuss 
this  hypothesis  satisfactorily  here  and  it  is  deferred  to  Chapter  VII, 
where  it  occupies  a  natural  place  in  our  account  of  the  mechanism  of 
differentiation. 

The  second  hypothesis  is  that  of  the  genetic  continuity  of  the  chromo- 
somes. The  essential  of  this  idea  is  that  the  chromosomes  which  appear 
during  the  preparation  for  a  mitosis,  are  definitely  related  in  a  precise 
way,  to  the  chromosomes  entering  that  nucleus  at  the  close  of  the 
preceding  division.  In  its  first  form  this  hypothesis  was  called  that 
of  the  individuality  of  the  chromosomes  (Rabl,  Boveri),  and  it  was 
held  that  the  chromosomes  actually,  though  not  visibly,  preserve 
their  structural  identity  during  the  period  of  interkinesis,  that  the 
chromosomes  of  one  mitosis  are  not  related  to  those  of  the  preceding 
division,  but  are  actually  the  same  chromosomes.  The  fact  that  little 
or  no  direct  evidence  of  chromosomal  identity  during  the  interkinesis, 
is  to  be  had,  has  led  to  the  remodeling  of  the  idea  of  individuality, 
into  the  hypothesis  of  genetic  continuity. 

The  nature  of  the  evidence  bearing  upon  this  hypothesis,  while  not 
scanty,  is  largely  circumstantial,  and  hardly  affords  definite  proof,  either 
affirmatively  or  negatively.  We  may  suggest  some  of  this  evidence 
without  pretending  to  give  a  detailed  account  of  the  facts. 

At  the  very  beginning  we  must  recognize  that  the  chromosomes  are 
actually  visible,  as  differentiated  structures,  only  during  that  com- 
paratively brief  period  of  cell  life  occupied  by  the  process  of  mitosis. 
Considering  first  some  of  the  facts  opposed  to  this  hypothesis,  we  should 
say  that  those  who  deny  the  fact  of  continuity,  maintain  that  during 
the  vegetative  period  of  cell  life  the  dissolution  and  disappearance  of  the 
chromosomes  is  not  only  apparent  but  real.  There  is  truly  little  visible 
and  direct  evidence  of  chromosomes  during  interkinesis.  And  further, 
much  new  chromatin  is  formed  during  this  period  which  cannot  be  dis- 
tinguished from  the  chromosomal  chromatin;  in  the  early  stages  of 
mitosis  much  chromatin  is  again  thrown  out  of  the  nucleus  and  takes 
no  part  in  the  formation  of  chromosomes.  It  is  impossible  to  say 
whether  the  chromatin  forming  the  chromosomes  is  or  is  not  then,  the 
same  as  that  previously  derived  from  them,  or  that  resulting  from  growth. 
Many  instances  are  known  where  the  chromatin  of  the  vegetative 
nucleus  is  nearly  all  contained  within  a  single  homogeneous  chromatin 


THE  CELL  AND  CELL  DIVISION 


71 


nucleolus  or  karyosome  (e.g.,  Asterias),  and  in  preparation  for  mitosis 
some  granules  pass  out  of  the  karyosome  and  form  into  distinct  chro- 
mosomes while  the  greater  part  of  the  karyosome  dissolves  (Fig.  35); 
it  is  difficult  to  understand  how  the  chromosomes  could  have  preserved 
their  integrity  through  such  a  history.  In  amitosis  no  chromosomes 
are  visible  yet  the  nuclei  of  cells  thus  formed  seem  to  function  normally 
and  such  cells  are  capable  of  typical  differentiations  and  in  a  few  in- 
stances are  said,  with  some  question,  however,  to  be  subsequently 
capable  of  division  by  mitosis,  and  of  normal  chromosome  formation. 


x_V\    ,.', 

"fe^^fe- 


FIG.  35. — Primary  oocyte  of  the  star-fish,  Asterias  forbesii,  at  beginning  of 
division.  From  Dahlgren  and  Kepner  (Jordan).  Chromatin  leaving  the  "  chro- 
matin reservoir"  or  chromatin  nucleolus,  and  being  added  to  the  chromosomes. 


Such  an  interruption  of  a  series  of  mitotic  divisions  by  a  period  of 
amitotic  division  would  seem  to  exclude  the  possibility  of  both  genetic 
relation  and  specificity  of  the  chromosomes.  In  those  forms  where 
chromosomes  are  not  normally  formed,  the  chromatin  granules  are  the 
units  in  nuclear  division;  and  even  when  these  are  formed  into  chromo- 
somes the  essential  step  in  nuclear  division  is  the  fission  of  these  granules, 
which  thus  seem  to  be  the  real  units  of  the  chromatic  substance.  Yet 
one  could  hardly  maintain  the  genetic  continuity  of  these  granules  upon 
other  than  logical  grounds,  and  to  many  there  seems  no  stopping  place 


72 


GENERAL  EMBRYOLOGY 


short  of  this,  if  the  fact  of  continuity  of  organized  chromatic  structure 
be  accepted  to  begin  with. 

On  the  other  hand,  those  who  adhere  to  the  continuity  hypothesis 
find  many  supporting  facts  in  the  phenomena  of  fertilization  and 
maturation,  the  importance  of  which  can  be  appreciated  more  fully 
after  our  consideration  of  these  subjects.  They  assume,  from  the  con- 
sideration of  the  behavior  of  the  chromosomes,  that  they  only  apparently 
lose  their  structural  and  functional  identity  during  the  interkinesis, 
and  that  something  directly  representative  of  the  chromosomal  structure 


FIG.  36. — Indications  of  the  individuality  of  the  chromosomes  in  the  cleavage 
of  the  egg  of  Ascaris.  From  Wilson,  "Cell,"  after  Boveri.  E.  Anaphase  of  first 
cleavage.  F.  Two-cell  stage  with  lobed  nuclei,  the  lobes  formed  by  the  ends  of  the 
chromosomes.  G.  Early  prophase  of  next  division.  Chromosomes  reforming, 
centrosomes  dividing.  H.  Late  prophases  of  same  division,  the  chromosomes 
lying  with  their  ends  in  the  same  position  as  at  the  close  of  the  preceding  division. 

is  present  in  the  resting  nucleus,  invisible  directly  and  known  only  from 
its  consequences.  There  are,  it  is  true,  a  few  instances  in  which  the 
chromosomal  arrangement  is  said  really  to  be  visible  in  the  interkinesis 
(Figs.  36,  37),  but  these  cases  are  not  very  clear,  except  in  the  maturation 
divisions  leading  directly  to  the  formation  of  the  specialized  germ  cells, 
where  the  chromosomes  are  definitely  known  to  be  directly  continuous. 
The  remarkable  constancy  of  form,  volume,  and  number  of  chromosomes 
throughout  the  cells  of  a  given  organism  and  species,  is  important  evi- 
dence favoring  the  hypothesis  under  consideration.  The  probability 
is  so  high  as  to  amount  almost  to  certainty,  that  if  the  chromosomes 


THE  CELL  AND  CELL  DIVISION 


73 


were  new  and  independent  formations  in  each  mitosis,  they  would  show 
a  normal  variability  in  number  throughout  long  series  of  mitoses. 
However,  numerical  variability  is  so  rare  as  to  be  practically  absent, 
although  the  few  exceptions  known  are  emphasized  by  those  who  do 
not  accept  the  idea  of  continuity.  Constancy  of  form  seems  to  be  less 
precise  than  constancy  of  volume,  but  both  are  sufficiently  marked  to 
be  noteworthy.  It  is  difficult  to  get  precise  observations  here  on  account 
of  the  liability  to  shrinkage  or  deformation  of  the  chromosomes  in  the 
preparation  of  the  material  for  study.  There  is  often  undoubtedly  a 
definite  pairing  of  chromosomes  in  the  somatic  nuclei  (Fig.  33),  while  in 

the  germ  nuclei,  with  =  chromosomes,  the  same  categories  of  chromo- 
somal form  and  size  found  in  somatic  nuclei  are  distinguishable,  but 
these  are  no  longer  paired — there  are 
only  single  representatives  of  each. 
This  is  one  of  the  strongest  points 
favoring  this  hypothesis.  And  no  less 
significant  is  the  fact  that  the  odd  or 
unpaired  chromosomes  associated  with 
sex  differentiation,  mentioned  above, 
remain  constant  in  size  and  form  and 
number,  throughout  the  tissue  cells  of 
certain  individuals,  and  are  easily  rec- 
ognizable, not  only  through  their  pe- 
culiar morphology  but  on  account  of 
their  peculiar  behavior  as  well.  It  is 

important    in  this  connection,  to   no- 

, .       , ,     ,  ,,  , .      ,         ,  FIG.    37. — Indications  of  the  in- 

tice  that  these  particular  chromosomes  dividuality  of  the  chromosomes  in 

may  remain   undissolved  even  in  the  early  prophase  of  division  of  a  sper- 

resting  nucleus,  where  they  had  often  matogonium  of  Brachystola  magna. 

.         ..      .  ,.  11.   From  Sut ton.     Spiremes  forming  in 

been  described   as  chromatm  nucleoli  lobes  of  the  nucieus  corresponding 

or  other  bodies,  before  their  signifi-  with  the  chromosomes  which  en- 
canoe  was  appreciated  or  their  history  £^3±£,"  ^d-eof  ti» 
known. 

Another  group  of  facts  of  quite  a  different  character  has  an  important 
bearing  upon  these  hypotheses.  It  sometimes  happens  in  mitosis  that 
one  or  more  chromosomes  belonging  to  one  daughter  group,  accidentally 
become  included  with  the  other  group  so  that  one  of  the  daughter 
nuclei  has  fewer,  the  other  more,  than  the  normal  somatic  number. 
In  subsequent  divisions  of  these  cells  the  number  of  chromosomes 
appearing  is  not  the  normal,  but  the  increased  or  diminished  number, 
the  sum  of  the  two,  however,  always  being  2s.  Or  in  fertilization  of  the 

egg  by  the  sperm,  each  of  which  has  -~  chromosomes,  various  abnor- 


74 


GENERAL  EMBRYOLOGY 


malities  occur  in  the  distribution  of  the  chromosomes,  and  it  is  always 
the  case  that  the  number  of  chromosomes  forming  out  of  a  nucleus  is  the 
same  as  the  number  passing  into  it,  no  matter  how  that  deviates  from  the 
normal  (Boveri)  (Fig.  38).  There  seems  to  be  no  regulation  within  the 
nucleus  in  this  respect,  such  as  would  result  were  it  a  unified  structure 
tending  always  to  maintain  its  own  normal.  In  many  instances  of 


D 


FIG.  38. — Indications  of  the  individuality  of  the  chromosomes  in  the  fertiliza- 
tion of  Ascaris.  From  Wilson,  "Cell,"  after  Boveri.  A.  The  two  chromosomes 
of  the  egg-nucleus,  accidentally  separated,  have  given  rise  each  to  a  reticular  nu- 
cleus (9  ,  9 ) ;  the  sperm-nucleus  below  (c?).  B.  Later  stage  of  the  same,  a  single 
chromosome  in  each  egg-nucleus,  two  in  the  sperm-nucleus.  C.  An  egg  in  which 
the  second  polar  body  has  been  retained;  p.b.*  the  two  chromosomes  arising  from 
it,  9  the  egg-chromosomes,  cT  the  sperm-chromosomes.  D.  Resulting  equator- 
ial plate  with  six  chromosomes. 


alternation    of   generations,  the  agametically  produced  generation  is 
formed  from  a  cell  with  -    chromosomes;  in  all  the  cells  of  such  an 

organism,   the    nuclei    show  the    same    reduced   number,  even  in    so 
complex   an   organism   as   the  fern   prothallus.     In   some  forms,   the 


THE  CELL  AND  CELL  DIVISION 


75 


chromosome  groups  derived  from  the  egg  and  sperm  nuclei,  each  of  ^ 

chromosomes,  remain  distinguishable  through  a  considerable  series  of 
the  early  divisions  of  the  zygote;  two  separate  spiremes,  each  forming 

-   chromosomes,    may  even  be    distinguished    sometimes    (Riickert). 
And  in  some  hybrids  where  the  chromosomes  are  unlike  in  number,  or 


FIG.  39. — The  chromosome  group  in  the  hybrids  of  the  Teleosts,  Fundulus  and 
Menidia,  showing  the  distinctness  of  the  paternal  and  maternal  elements.  From 
Moenkhaus.  A.  Late  anaphase  of  first  cleavage  of  normally  fertilized  Fundulus. 
All  chromosomes  of  the  long  type.  B.  Anaphase  of  first  cleavage  of  normally 
fertilized  egg  of  Menidia.  All  chromosomes  of  the  short  type.  C.  Anaphase  of 
first  cleavage  of  Fundulus  egg  fertilized  with  Menidia  sperm.  To  the  left  long 
(Fundulus)  chromosomes  only,  to  the  right  short  (Menidia)  chromosomes  only. 

D.  Anaphase  of  first  cleavage  of  Menidia  egg  fertilized  with  Fundulus  sperm. 

E.  Metaphase  of  first  cleavage  of  Fundulus  egg  fertilized  with  Menidia  sperm. 
cT,  chromosomes  of  paternal  origin.     9  ,  chromosomes  of  maternal  origin. 


form,  or  size,  the  two  groups  derived  from  the  male  and  female  parents 
remain  distinct  (Fig.  39),  often  for  a  very  long  time,  perhaps  even 
throughout  life  (Moenkhaus,  Herbst,  Baltzer). 

While  evidence  of  the  kind  suggested  above  does  not  constitute 
definite  proof  of  the  genetic  continuity  of  the  chromosomes,  it  is  very 
difficult  to  explain  the  facts  upon  any  other  basis.  In  the  absence  of  any 
other  satisfactory  hypothesis  in  this  field  it  seems  wise  to  accept  a 
certain  modification  of  the  essential  idea,  as  a  working  basis,  while 
admitting  the  difficulties  of  demonstration  and  the  existence  of  some 
apparent  contradictions.  We  may  recognize  in  this  difference  of  opinion 


76  GENERAL  EMBRYOLOGY 

regarding  the  continuity  of  the  chromosomes,  an  outcropping  of  the 
opposed  preformational  and  epigenetic  conceptions,  which  pervade  all 
descriptions  of  developmental  phenomena.  The  hypothesis  of  chromo- 
somal continuity  is  essentially  a  preformational  view;  those  who  deny 
continuity  assume  the  epigenetic  view. 

Perhaps  the  immediate  solution  of  the  difficulty  here  may  not  be  un- 
like the  solution  of  the  greater  problem  and  more  fundamental  difference 
of  opinion.  It  seems  likely  that  what  is  directly  continuous  from 
nucleus  to  nucleus,  i.e.,  what  is  preformed,  is  some  sort  of  fundamental 
organization  determining  the  chromosomal  structure  of  the  nucleus, 
just  as  the  "organization"  of  the  egg  determines  the  structure  of  the 
embryo  developed  from  it.  Yet  what  appears  is  a  new  structure,  formed 
epigenetically  under  or  through  the  influence  of  the  " organizational" 
factor,  by  the  material  present,  and  subject  to  the  modifying  influences 
of  changing  conditions  external  to  the  nucleus.  That  is  to  say,  the 
formation  of  the  chromosomes  out  of  the  chromatin  of  the  vegetative 
nucleus  is  to  be  regarded  as  a  true  process  of  development.  The  reac- 
tion between  the  fundamental  organized  structure  of  the  nucleus,  and 
the  stimuli  acting  upon  it,  consists  in  the  formation  of  the  chromosomes 
and  other  structures,  not  to  be  seen  in  the  nucleus  previous  to  this 
reaction.  The  chromosomes  are  thus  no  more  genetically  continuous 
than  the  organs  of  adults,  and  yet  there  is  a  real  continuity  of 
organization  underlying. 

In  many  respects  the  nucleus  is  analogous  to  an  organism,  the  chro- 
mosomes and  other  nuclear  structures  representing  the  organismal 
organs;  both  are  functionally  specific,  are  constant  in  number,  form, 
and  size  throughout  the  species;  both  reproduce  and  exhibit  develop- 
ment as  a  form  of  response.  As  Wilson  points  out,  the  analogy  is  far 
from  complete — no  complete  analogy  is  known,  but  that  there  is  an 
underlying  organization  of  some  kind,  continuous  and  specific,  seems 
clear  although  we  remain  entirely  ignorant  of  what  it  really  is,  and  just 
how  it  operates  and  is  affected  by  new  conditions. 

Before  leaving  the  subjects  of  the  cell  and  cell  division  we  must  con- 
sider briefly  two  other  questions  which  are  of  great  importance,  but 
which  are  also  still  in  a  hypothetical  state.  These  are,  the  nature  of 
the  causes  leading  to  cell  division,  and  the  nature  of  the  fundamental 
mechanism  of  the  process.  The  interactions  between  the  nucleus  and 
cytoplasm,  and  between  these  and  the  external  medium,  which  consti- 
tute the  life  of  the  cell,  are  largely,  probably  wholly,  of  a  physico-chem- 
ical nature.  As  such  their  normal  procedure  is  dependent  upon  certain 
mass  relations  of  the  interacting  substances,  and  upon  the  maintenance 
of  adequate  pathways  of  interaction  between  them.  For  these  reasons 
we  look  quite  naturally,  in  searching  for  a  possible  cause  of  cell  division, 
to  the  volumetric  relations  of  the  nucleus  and  cytoplasm,  and  to  the 


THE  CELL  AND  CELL  DIVISION  77 

extent  of  the  surface  of  these  parts  of  the  cells  in  relation  to  the  two 
masses. 

Immediately  after  mitosis  is  completed  the  nucleus  grows  very  rapidly 
for  a  brief  period,  and  then  much  more  slowly  or  not  at  all.  The  cyto- 
plasm, however,  does  not  show  this  rapid  initial  growth,  but  maintains 
a  fairly  uniform  and  continuous  growth  rate.  As  a  result,  in  a  newly 
formed  cell  the  ratio  of  nuclear  mass  to  cytoplasmic  mass  becomes  quite 
high,  but  soon  diminishes,  and  in  an  older  cell  diminishes  rapidly,  since 
the  cytoplasm  continues  to  grow  after  the  nucleus  nearly  ceases.  Con- 
sidering first  the  relation  of  the  surface  to  the  mass,  and  assuming  for 
illustration  that  the  cell  and  nucleus  are  both  spherical,  we  can  see  how 
this  increase  in  size  alters  the  relation  of  mass  to  path  of  interchange, 
since  the  area  of  the  surface  of  an  enlarging  sphere  increases  relatively 
slower  than  the  volume.  Should  the  cell  double  its  diameter  through 
growth,  it  increases  its  volume  eight  times  and  its  surface  only  four  times. 
A  cubical  cell  doubling  the  length  of  its  sides  reduces  its  ratio  of  area  to 
volume  as  2  :  1.  Or  expressing  a  similar  relation  somewhat  differently, 
a  spherical  cell  doubling  its  volume,  lessens  its  ratio  of  surface  to  volume 
approximately  as  5  :  4,  while  a  cubical  cell  doubling  its  volume  reduces 
the  same  relation  approximately  as  6  :  4.75. 

These  relations  are  probably  important  for  both  of  the  chief  interac- 
tions of  cell  life,  those  between  nucleus  and  cytoplasm,  and  between 
cytoplasm  and  surrounding  medium.  The  only  pathway  between  cyto- 
plasm and  nucleus  is  the  nuclear  membrane,  while  the  surface  of  the 
cytoplasm  or  cell  wall  forms  the  pathway  between  cell  and  medium. 
There  is  of  course  a  limit  to  the  capacity,  so  to  speak,  of  these  surfaces, 
and  as  the  cell  increases  in  volume  this  limit  tends  to  be  reached.  The 
ratios  of  these  surfaces  to  the  masses  are  raised  by  the  division  of  the 
cell,  which  reduces  volume  more  than  surface,  and  thus  restores  the 
efficiency  of  the  surfaces  as  pathways  of  interaction.  The  tendency  for 
the  cytoplasmic  mass  to  increase  more  rapidly  than  the  mass  of  the 
nucleus  in  older  cells  seems  to  be  of  even  greater  importance  than  these 
surface  to  mass  relations.  There  seems  to  be  a  fairly  definite  specific 
limit  to  the  ratio  of  nuclear  mass  to  cytoplasmic  mass,  although  it  is 
difficult  to  say  whether  after  all  the  nuclear  surface  relation  is  not  even 
here  an  equally  important  factor.  This  mass  relation  is  called  the 
"kern-plasma"  or  nucleo-cytoplasmic  relation,  the  importance  of  which 
is  emphasized  particularly  by  Richard  Hertwig.  There  is  a  definite 
average  cell  size  in  a  given  tissue  and  species ;  a  large  or  a  small  organ 
or  organism  does  not  possess  respectively  larger  or  smaller  cells,  but 
larger  or  smaller  numbers  of  cells  of  the  same  average  dimensions. 
The  limiting  factor,  however,  seems  to  be  not  the  actual  bulk  of  the  cell, 
but  the  proportion  between  the  volume  of  the  nucleus  and  that  of  the 


78 


GENERAL  EMBRYOLOGY 


cytoplasm,  i.e.,  (y~)  •  In  many  cells  during,  and  immediately  after, 
division  the  nucleus  grows  at  the  expense  of  the  cytoplasm,  and  the 
ratio  ly^J  is  raised  (Fig.  40).  The  cell  then  enters  upon  its  vegetative 

phase,  during  which  the  cytoplasm  grows  more  rapidly  than  the  nucleus, 
and  the  ratio  diminishes  toward  the  lower  functional  limit ;  as  the  ratio 
approaches  this  limit  the  functional  activities  of  the  cell  change,  and  the 
normal  vegetative  processes  give  place  to  that  form  of  action  which  we 
call  cell  division,  during  which  the  ratio  again  rises  to  a  value  permitting 
normal  vegetative  functioning.  It  is  not  yet  possible  to  state  in 
precise  quantitative  terms  what  the  limits  of  these  ratios  of  volume 


2 

1.8 


i  b 


1.4 


C 

1.9 


1.4 


10 


16       11 


FIG.  40. — Curve  showing  the  increase  in  volume  of  nucleus  (a-d)  and  cytoplasm 
(6-c)  during  interkinesis,  in  the  Infusorian;  Frontonia  leucas,  at  temperature  of 
25°  C.  After  Popoff.  Ordinates,  volume;  abcissas,  time  in  hours.  Each  curve 
shows  a  doubling  of  volume  (1:2)  during  the  seventeen  hour  period  of  the  inter- 
kinesis.  Each  curve  is  based  upon  its  own  units  of  measurement,  which  are 
different  for  the  two  curves.  The  nucleo-cytoplasmic  relation  is  identical  at  the 
beginning  and  end  of  the  period.  The  size  of  the  nucleus  is  relatively  smallest  at 
fifteen  hours;  then  the  nucleus  begins  to  grow  very  rapidly,  so  that  at  the  time 
of  the  division  of  the  whole  cell,  the  original  relation  is  restored. 

and  surface  are  in  specific  instances,  nor  even  to  say  whether  the  volume 
or  surface,  or  volume  and  surface  relations  are  those  essentially  involved. 
But  so  far  this  nucleo-cytoplasmic  hypothesis  is  the  most  plausible  ex- 
planation of  the  nature  of  the  immediate  conditions  of  cell  division. 
It  should  be  said,  however,  that  the  applicability  of  Hertwig's  "  Kern- 
plasma  Relation"  is  still  chiefly  limited  to  Protozoan  cells,  and  that 
even  here  there  are  many  contradictions.  Some  of  the  more  obvious 
exceptions  to  th'e  definition  of  such  a  limiting  ratio  are  to  be  ex- 
plained as  special  adaptations.  Such  are  the  very  great  cytoplas- 
mic  bulk  of  many  egg  cells  or  the  relatively  large  size  of  the  nucleus 
in  the  sperm  cell.  Many  other  exceptions  of  special  character  are  to 


THE  CELL  AND  CELL  DIVISION 


79 


be  found,  usually  associated  with  reproductive  processes,   e.g.,  brood 
formation. 

Regarding  the  real  nature  of  the  mechanism  of  mitosis,  even  less 
can  be  said  to  be  known  than  with  respect  to  its  causes.  The  arrange- 
ment of  the  achromatic  amphiaster  and  the  behavior  of  the  chromosomes 
show  that  in  the  mitotically  dividing  cell  the  forces  or  tensions  are 


A 


FIG.  41. — Diagrams  of  the  arrangement  of  the  spongioplasmic  network 
(mitome)  in  the  cell.  A,  B,  C,  from  Korschelt  and  Heider.  after  Heidenhain. 
A.  Schema  of  the  arrangement  of  the  spongioplasm  as  it  would  appear  in  the 
absence  of  a  nucleus.  Symmetrical  monocentric  system.  B.  Arrangement  in 
the  presence  of  a  nucleus.  Asymmetrical  monocentric  system.  C.  Arrange- 
ment at  the  beginning  of  mitosis.  Commencement  of  dicentric  system.  D. 
Arrangement  during  the  process  of  mitosis.  Symmetrical  dicentric  system. 
a,b,  cell  axis;  k,  nucleus. 


arranged  in  a  dicentric  system,  whereas  in  the  vegetative  cell  the  system 
is  monocentric  (Fig.  41).  As  the  result  of  the  action  of  the  forces  in  this 
dicentric  system  the  chromosome  halves  are  separated  and  the  cyto- 
plasm divided.  The  problem  here  is  to  discover  the  nature  of  the 
forces  and  the  cause  of  the  formation  of  the  amphiaster  in  the  form 
which  it  has.  In  explanation  of  the  first  part  of  the  problem,  i.e.,  the 


80  GENERAL  EMBRYOLOGY 

divergence  of  the  chromosomes,  it  was  formerly  believed  that  the  fibers 
in  the  amphiaster  were  the  active  elements  and  that  different  groups  of 
these  fibers  had  different  functions.  Thus  the  mantle  fibers  attached 
to  the  chromosomes  were  supposed  to  be  contractile  and  by  shortening 
to  draw  the  chromosomes  toward  the  ends  of  the  spindle,  the  central 
spindle  remaining  rigid  and  resisting  any  tendency  for  the  ends  of  the 
spindle  to  approach  as  the  result  of  the  contraction  of  the  mantle  fibers, 
and  at  the  same  time  serving  as  a  sort  of  track  upon  which  the  chromo- 
somes would  slide  along.  The  asters  then,  either  as  anchors  helped  to 
fix  the  ends  of  the  spindle,  or  by  contraction  served  to  draw  the  fully 
diverged  chromosomes  further  into  the  daughter  cells  (Fol,  Van  Bene- 
den,  Heidenhain).  This  naive  explanation  cannot  be  applied  in  toto 
to  any  known  instance  of  division,  although  certain  features  may  be 
correct  descriptions  of  the  events  in  certain  forms.  And  there  are  many 
facts  opposing  such  an  account  of  the  action  of  the  forces  of  mitosis,  such 
as  the  absence  of  mantle  fibers  or  of  asters  in  many  mitoses.  Another 
hypothesis,  in  greater  favor  at  present,  and  apparently  well  founded,  is 
that  the  centrosomes  and  centrospheres  rather  than  the  achromatic 
fibers  are  the  active  elements,  and  further,  that  their  activity  is  primarily 
of  a  chemical  nature.  Thus  the  chromosomes  are  believed  to  be  chem- 
ically attracted  toward  the  centrosomes,  the  achromatic  fibers  being 
passive  and  formed  merely  as  the  result  of  the  rearrangement  of  cyto- 
plasmic  granules  along  the  paths  of  the  chemical  transformations  which 
have  their  seat  in  the  centrosomes  and  extend  thence  through  the  cyto- 
plasm (Strasburger) .  This  hypothesis  goes  farther  and  makes  it  possible 
to  explain  the  division  of  the  cytoplasm  on  the  same  basis.  For  the 
chemical  transformations  centering  in  the  centrosomes  might  easily 
influence  the  tensions  of  the  comparatively  impermeable  surface  film 
of  the  cell  so  that  in  the  region  near  the  centrosomes  the  tensions  would 
be  increased  while  that  region  farthest  from  the  centrosomes  and  sym- 
metrically related  to  them  both,  namely,  the  plane  of  the  equatorial 
plate,  would  be  a  region  of  lower  tension;  the  result  of  this  would,  of 
course,  be  a  constriction  in  this  plane.  It  is  quite  likely  too  that  dif- 
ferences in  electric  tension  accompany  these  chemical  transformations, 
and  these  might  assist  in  the  alteration  of  the  surface  tensions  in  such  a 
way  as  to  contribute  to  the  same  end  (R.  S.  Lillie).  It  is  known  that 
there  are  differences  in  the  electric  potential  in  different  regions  of  the 
dividing  cell,  in  some  cases  at  least. 

A  further  development  of  the  chemical  hypothesis  attempts  to  explain 
the  formation  of  the  amphiaster  itself.  Thus,  increase  in  the  ratio  of 
the  volume  of  the  cytoplasm  to  volume  or  surface  of  the  nucleus  beyond 
a  certain  point  leads  to  a  chemical  alteration  of  the  centrosomes  such 
that  they  become  the  centers  of  two  equivalent  series  of  chemical  reac- 
tions with  the  cytoplasm,  the  result  of  which  is  the  formation  of  the 


THE  CELL  AND  CELL  DIVISION  81 

dicentric  system.  It  is  suggested,  reasonably,  that  the  chemical  altera- 
tion is  such  that  the  centrosomes  absorb  or  condense  the  more  liquid 
parts  of  the  cytoplasm,  leaving  this  considerably  more  dense  than  in  the 
vegetative  condition  (Biitschli,  Rhumbler).  The  existence  of  two  such 
centers  withdrawing  the  more  liquid  parts  of  the  cytoplasm  would 
lead  to  the  radiations  seen  in  the  asters  and  spindle,  which  would  thus  re- 
sult from  a  physical  alteration  of  the  structure  of  the  cytoplasm  induced 
by  the  chemical  changes  within  the  centrosomes.  This  is  all  extremely 
hypothetical  of  course,  but  there  are  many  inorganic  phenomena  as 
well  as  processes  to  be  seen  in  the  cleavage  of  the  egg  wThich  lend  con- 
siderable support  here.  At  any  rate  these  hypotheses  represent  the 
state  of  our  present  ignorance  of  the  nature  and  origin  of  the  mitotic 
figure  and  process.  There  are  many  reasons  for  believing  that  the  chem- 
ical differentiations  within  the  cell  are  of  fundamental  importance  here, 
such  as  the  fact  that  cells  can  be  made  to  divide  artificially  by  altering 
their  chemical  structure.  And  that  interactions  of  the  nucleus  and 
cytoplasm  are  involved  is  indicated  by  the  important  observation  that 
while  the  amphiaster  may  be  formed  in  the  absence  of  a  nucleus,  no 
real  division  of  the  cell  may  occur  without  the  presence  of  nuclear 
material  (Boveri,  Ziegler). 


REFERENCES  TO  LITERATURE 

VON  BAER,  K.  E.,  Die  Metamorphose  des  Eies  der  Batrachier  vor  der 
Erscheinung  des  Embryo  und  Folgerungen  aus  ihr  fur  die  Theorie 
der  Erzeugung.  Arch.  Anat.  u.  Phys.  1834. 

BALTZER,  F.,  Zur  Kenntnis  der  Mechanik  der  Kerntheilungsfiguren. 
Arch.  Entw.-Mech.  32.  1911. 

BENDA,  C.,  Die  Mitochondria.  Ergebnisse  Anat.  u.  Entw.  12.  1903 
(1902). 

VAN  BENEDEN,  E.,  Recherches  sur  la  composition  et  la  signification  de 
Pceuf,  etc.  Mem.  Acad.  roy.  Belgique.  34.  1870.  Recherches 
sur  la  maturation  de  Pceuf  et  la  fecondation.  Arch.  Biol.  4. 
1883. 

BONNEVIE,  K.,  Chromosomenstudien.     Arch.  Zellf.     1.     1908. 

BOVERI,  T.,  Zellenstudien  II.  Die  Befruchtung  und  Teilung  des  Eies 
von  Ascaris  megalocephala.  Jena.  Zeit.  22  (16).  1888.  Zel- 
lenstudien III.  Ueber  das  Verhalten  der  chromatischen  Kern- 
substance  bei  der  Bildung  der  Richtungskorper  und  bei  der 
Befruchtung.  Jena.  Zeit.  24  (17).  1890.  Ueber  die  Befruch- 
tungs-  und  Entwickelungsfahigkeit  kernloser  Seeigel-Eier,  etc. 
Arch.  Entw.-Mech.  2.  1895.  Zur  Physiologic  der  Kern-  und 
Zelltheilung.  Sitz.-Ber.  Phys.-Med.  Ges.  Wtirzburg.  1897. 


82  GENERAL  EMBRYOLOGY 

BIJTSCHLI,  0.,  Untersuchungen  liber  mikroskopische  Schaume  und  das 

Protoplasma.     Leipzig.     1892. 
CALKINS,  G.  N.,  Nuclear  Division  in  Protozoa.     Woods  Holl  Biol.  Lect. 

1899.     (See  also  ref.  Ch.  I.) 
CALKINS,  G.  N.,  and  CULL,  S.  W.,  The  Conjugation  of  Parama3cium  au- 

relia  (caudatum).     Arch.  Protist.     10.     1907. 

CONKLIN,  E.  G.,  Karyokinesis  and  Cytokinesis  in  the  Maturation,  Ferti- 
lization and  Cleavage  of  Crepidula  and  other  Gasteropoda.     Jour. 

Acad.  Nat.  Sci.  Philadelphia.     12.     1902.     Cell  Size  and  Nuclear 

Size.     Jour.  Exp.  Zool.     12.     1912. 
DAHLGREN,  U.,  and  KEPNER,  W.  A.,  Text-book  of  the  Principles  of 

Animal,  Histology.     New  York.     1908. 
DELAGE,  Y.,  La  structure  du  protoplasma  et  les  theories  sur  Theredite  et 

les  grands  problems  de  la  Biologic  generate.     Paris.     1895. 
DELLA  VALLE,  P.,  L'Organizzazione  della  chromatina'studiata  mediante 

il   numero   dei    chromosomi.     Archiv.     Zoologico.     (Napoli.)     4. 

1909. 
DOBELL,  C.   C.,  Chromidia  and  the  Binuclearity  Hypotheses:  a  Review 

and  a  Criticism.     Q.  J.  M.  S.    53.     1909. 
DOFLEIN,  F.     (See  ref.  Ch.  I.) 
DUESBERG,  J.,  Nouvelles  recherches  sur  Tappareil   mitochondrial  des 

cellules  seminales.     Arch.  Zellf.     6.     1910. 
ERDMANN,  R.,  Kern-  und  Protoplasmawachsthum  in  ihren  Beziehungen 

zueinander.     Anat.  Hefte.     18.     1910. 
ERHARD,  H.,  Studien  liber  "Trophospongien."     Zugleich  ein  Beitrag 

zur    Kenntnis   der    Secretion.     Festschr.    f.    R.    Hertwig.     Jena. 

1910. 
FARMER,  J.  B.,  The  Structure  of  Animal  and  Vegetable  Cells.     Lan- 

kester's  Treatise  on  Zoology.     I,  2.     London.     1902. 
FICK,  R.,  Vererbungsfragen,  Reduktions-  und  Chromosomenhypothesen 

Bastardregeln.     Ergebnisse  Anat.  u.  Entw.     16.     1906  (1907). 
FLEMMING,  W.,  Zellsubstanz,  Kern  und  Zellteilung.     Leipzig.     1882. 
GURWITSCH,  A.,  Morphologic  und  Biologie  der  Zelle.     Jena.     1904. 
HACKER,  V.,  Praxis  und  Theorie  der  Zellen-  und  Befruchtungslehre. 

Jena.     1899. 
HARTMANN,  M.,  Die  Konstitution  derProtistenkerne  und  ihre  Bedeutung 

fur  die  Zellenlehre.     Jena.     1911. 
HEIDENHAIN,    M.,    Cytomechanische   Studien.     Arch.  Entw.-Mech.  1. 

1895.     Plasma  und  Zelle.     Jena.     1907. 

HERTWIG,  O.,  Die  Zelle  und  die  Gewebe.     Jena.     I,  1893.     II,  1898. 
HERTWIG,  R.,  Ueber  neue  Probleme  der  Zellenlehre.     Arch.  Zellf.  1. 

1908. 
JORGENSEN,    M.,   Zur   Entwicklungsgeschichte  des   Eierstockeies  von 


THE  CELL  AND  CELL  DIVISION  83 

Proteus    anguineus     (Grottenholm) .     Festschr.     f.     R.     Hertwig. 

Jena.     1910. 

LILLIE,  F.  R.,  Karyokinetic  Figures  of  Centrifuged  Eggs.     An  Experi- 
mental Test  of  the  Center  of  Force  Hypothesis.     Biol.  Bull.     17. 

1909. 
LILLIE,  R.  S.,  The  Physiology  of  Cell-division.     IV.    The  Action  of 

Salt  Solutions  followed  by  hypertonic  Sea-water  on  unfertilized 

Sea-urchin  Eggs,  and  the  Role  of  Membranes  in  Mitosis.     Jour. 

Morph.     22.     1911. 
METCALP,  M.  M.,  Opalina.    Its  Anatomy  and  Reproduction,  with  a 

Description  of  Infection  Experiments  and  a  Chronological  Review 

of  the  Literature.     Arch.  Protist.     13.     1909. 
MOENKHAUS,  W.  J.,  The  Development  of  the  Hybrids  between  Fundulus 

heteroclitus  and  Menidia  notata,  with   especial   Reference  to  the 

Behavior  of  the  Maternal  and  Paternal  Chromatin.     Am.  Jour. 

Anat.     3.     1904. 
MONTGOMERY,  T.  H.,  JR.,  A  Study  of  the  Chromosomes  of  the  Germ 

Cells  of  the  Metazoa.    Trans.  Amer.  Phil.  Soc.     20.     1901.     On 

the  Dimegalous  Sperm  and  Chromosomal  Variation  of  Euschistus, 

with    Reference    to    Chromosomal    Continuity.     Arch.    Zellf.     6. 

1910. 
NOWIKOFF,  M.,  Zur  Frage  iiber  die  Bedeutung  der  Amitose.     Arch. 

Zellf.     5.     1910. 

POPOFF,  M.,  Experimented  Zellstudien.     Arch.  Zellf.     1.     1908. 
PRENANT,  A.,  Theories  et  interpretations  physiques  de  la  mitose.    Jour, 

Anat.  Phys.     Paris.     46.     1910.     Les  mitochondries  et  1'ergasto- 

plasme.     Id.     46.     1910. 
PRENANT,  A.,  BOUIN,  P.,  et  MAILLARD,  L.,  Traite*  d'Histologie.    Paris. 

1910. 
PRZIBRAM,  H.,  Experimental  Zoology.     I.     Embryogeny.    Cambridge. 

1908.     Anwendung     elementarer     Mathematik     auf    biologische 

Probleme.     Leipzig.     1908. 
SCHAXEL,   J.,   Die   Morphologic  des    Eiwachstums  und   der  Follikel- 

bildungen  bei  den  Ascidien — Ein  Beitrag  zur  Frage  der  Chromidien 

bei  Metazoen.     Arch.  Zellf.     4.     1910. 
SCHEWIAKOFF,    W.,    Ueber    die    karyokinetische     Kerntheilung    der 

Euglypha  alveolata.     Morph.  Jahrb.     13.     1887. 
SCHNEIDER,  K.  C.,  Histologisches  Praktikum  der  Tiere.     Jena.     1908. 

Histologische     Mitteilungen.     III.      Chromosomengenese.     Fest- 
schr. f.     R.  Hertwig.     Jena.     1910. 

STRASBURGER,  E.,  Zellbildung  und  Zellteilung.     3  Auf.     Jena.     1880. 
SUTTON,   W.  S.,   On  the  Morphology  of  the  Chromosome  Group  in 

Brachystola  magna.     Biol.  Bull.     4.     1902. 


84  GENERAL  EMBRYOLOGY 

VIRCHOW,  R.,  Die  Cellularpathologie  in  ihrer  Begriindung  auf  physiolog- 
ische  und  pathologische  Geweblehre.  Berlin.  1858. 

WILSON,  E.  B.,  Atlas  of  Fertilization  and  Karyokinesis.  New  York. 
1895.  On  Protoplasmic  Structure  in  the  Eggs  of  Echinoderms  and 
some  other  Animals.  Jour.  Morph.  15,  Suppl.  1899.  The  Cell 
in  Development  and  Inheritance.  Columbia  Univ.  Biol.  Ser.  IV. 
2  ed.  New  York.  1900. 

ZIEGLER,  H.  E.,  Experimentelle  Studien  liber  die  Zelltheilung.  II. 
Furchung  ohne  Chromosomen.  Arch.  Entw.-Mech.  6.  1898. 


CHAPTER  III 
THE  GERM  CELLS  AND  THEIR  FORMATION 

THE  reproductive  elements  of  the  Metazoa  are  single  cells, 
often  greatly  specialized  in  form,  and  always  highly  differen- 
tiated in  internal  structure  and  in  function.  They  differ  from 
the  reproductive  cells  of  several  groups  of  the  Metaphyta,  in 
that  they  cannot  function,  i.e.,  develop,  until  two  single  cells, 
usually  derived  from  two  different  individuals,  shall  have  met 
and  fused,  or  conjugated.  In  many  plants  single  reproductive 
cells  (spores)  are  formed  which  develop  directly  without  any 
such  conjugation,  and  which  are  therefore  to  be  distinguished 
from  the  true  germ  cells,  or  gametes,  which  develop  only  after 
conjugation.  We  shall  describe  the  process  of  conjugation,  or 
fertilization,  in  a  following  chapter,  but  in  order  to  appreciate 
the  significance  of  many  of  the  details  of  germ-cell  form  and 
structure,  we  must  remember  that  they  are  adapted  toward 
ensuring  the  conjugation  of  two  unlike  cells,  egg  and  sper- 
matozoon. 

Reproductive  cells  are  set  apart  from  vegetative  cells  in 
many  of  the  colonial  Protozoa.  In  some  cases  they  are  dis- 
tinguishable only  at  certain  times,  when  cells  usually  vegetative 
may  give  up  such  characteristics  and  become  reproductive;  in 
others  the  reproductive  and  vegetative  cells  remain  perma- 
nently distinct  though  only  slightly  differentiated  structurally. 
Finally  in  a  few  colonial  Protozoa  the  reproductive  cells  are 
considerably  modified  from  the  vegetative  condition,  and  in 
form  and  composition,  as  well  as  in  function  and  behavior,  are 
readily  distinguished  as  germ  cells.  Many  of  the  details 
regarding  these  cells  have  already  been  mentioned,  others  can 
be  more  conveniently  and  more  significantly  considered  later, 
in  connection  with  the  process  of  fertilization  (Chapter  V).  In 

85 


86  GENERAL  EMBRYOLOGY 

this  chapter  we  shall  first  describe  the  form  and  structure  of  the 
typical  germ  cells  of  the  Metazoa,  as  they  appear  when  fully 
formed  and  ready  to  function,  that  is,  just  prior  to  fertilization. 
We  shall  then  mention  some  of  the  more  important  modifica- 
tions of  structure  shown  in  different  groups,  and  finally  give  a 
brief  account  of  the  formation  and  history  of  the  germ  cells  up 
to  the  time  when  they  are  ready  actually  to  enter  upon  the  real 
process  of  development.  The  details  of  certain  phases  in  the 
history  of  the  germ-cell  nuclei,  namely,  the  maturation  processes, 
are  of  such  importance  that  we  shall  refer  to  them  only  briefly 
in  this  chapter  and  devote  that  following  to  a  more  extended 
account  of  these  events. 

Among  the  Metazoa  the  fully  formed  germ  cells  are  always 
of  two  very  unlike  types,  the  ova  or  eggs  and  the  spermatozoa 
or  sperm  cells.  These  cells  are  alike  only  with  respect  to  their 
nuclear  structure  and  composition.  Their  form  differences 
are  associated  with  fundamental  differences  in  function.  The 
egg  cell  contains  by  far  the  greater  share  of  the  substance  which 
is  to  form  the  material  basis  of  the  new  individual.  The  sperm, 
on  the  other  hand,  contributes  little  substance,  and  that  chiefly 
nuclear,  to  the  new  individual.  One  of  its  important  functions 
seems  to  be  that  of  ensuring  to  the  comparatively  passive  egg, 
a  stimulus  which  leads  it  to  react,  i.e.,  to  commence  develop- 
ment; and  the  sperm's  nuclear  configuration,  with  that  of  the 
egg,  together  appear  to  determine  the  course  of  development 
to  a  large  extent,  if  not  wholly. 

The  substance  of  which  the  ovum  is  composed  is  not  a  homo- 
geneous protoplasm.  The  cytoplasm  is  differentiated  and 
organized  into  a  definite  structural  and  chemical  (energetic) 
configuration.  The  details  of  this  configuration  are  uniform 
in  the  eggs  of  each  animal  kind,  i.e.,  it  is  specific.  This  cy to- 
pi asmic  structure  of  the  ovum,  although  itself  apparently 
determined  primarily  by  nuclear  activity,  is  of  great  importance 
in  maintaining  the  continuity  and  uniformity  of  organismal 
characteristics  through  successive  generations  (heredity). 

The  ova  are  less  modified  in  external  form  than  the  sperma- 
tozoa, and  often  approach  the  form  of  a  typical  cell,  except 


GERM  CELLS 'AND  THEIR  FORMATION 


87 


that  they  are  nearly  always  larger  than  ordinary  cells  (Fig.  42). 
The  size  of  the  ovum  is  not  related  to  the  size  of  the  organism 
producing  it,  but  is  in  general  related  to  the  amount  of  food 
substance  stored  in  it,  the  actually  living  protoplasm  showing 
much  less  variation  in  amount  than  the  deutoplasm.  The 
smallest  eggs  are  those  of  the  Mammals  (Figs.  43,  14,  VII),  in 
man  only  0.25  mm.  (250  micra  or  0.01  inch)  in  diameter,  others 
being  still  smaller — 0.07-0.10  mm.  (70-100  micro)  in  the  deer, 


v.ex. 


FIG.  42. — A.  Section  through  the  egg  of  the  lamprey,  Petromyzon  fluviatilis. 
After  Herfort.  B.  Spermatozoon,  drawn  to  scale,  d.en.,  dense  endoplasm; 
t'.ra.,  inner  membrane  (?  vitelline) ;  o.m.,  outer  membrane  (?  chorion) ;  p,  granular 
"polar  plasm;"  v.en.,  vacuolated  endoplasm;  v.ex.,  vacuolated  exoplasm;  /,  first 
polar  body;  //,  second  polar  spindle. 

and  only  0.065  mm.  (65  micro)  in  the  mouse.  The  largest  eggs 
are,  in  volume,  the  largest  known  cells;  such  are  the  "  yolks"  of 
birds'  eggs,  the  largest  of  which  are  several  inches  in  diameter 
and  equalled  in  other  groups  only  by  the  eggs  of  one  of  the 
sharks  (Heterodontus)  which  are  nearly  2  inches  (4.0  to  5.0  cm.) 
in  diameter.  In  a  very  few  cases  (some  Coelenterates  and 
Porifera,  and  a  few  worms)  the  ova  may  be  capable  of  loco- 
motion, performing  amoeboid  movements  (Fig.  44),  but  in 
nearly  all  cases  they  are  quiescent,  passive  structures  although 


88 


GENERAL  EMBRYOLOGY 


FIG.  43. — Fully  grown  human  oocyte  freshly  removed  from  the  ovary.  Out- 
side the  oocyte  are  the  clear  zona  pellucida  and  the  follicular  epithelium  (corona 
radiata).  The  central  part  of  the  oocyte  contains  deutoplasmic  bodies  and  the 
eccentric  nucleus  (germinal  vesicle) ;  superficially  is  a  well  marked  exoplasmic  or 
cortical  layer.  From  Waldeyer-Hertwig. 


FIG.  44. — Fully  grown  egg  of  Hydra  mridis,  containing  nuclei  of  ingested  cells. 
gv,  nucleus  of  egg.  The  egg  is  amceboid.  From  Waldeyer-Hertwig,  after 
Kleinenberg. 


GERM  CELLS  AND  THEIR  FORMATION  89 

containing  a  great  deal  of  potential  energy  which  becomes 
kinetic  after  fertilization,  during  the  early  stages  of  develop- 
ment . 

Upon  examining  the  internal  structure  of  the  egg  we  find 
that  the  nucleus  is  unusually  large  in  most  cases,  spherical  or 
ovoid,  and  with  or  without  a  nuclear  membrane  (Figs.  43,  45). 
It  is  located  centrally  or  eccentrically,  usually  the  latter  if  the 
egg  contains  an  appreciable  amount  of  food  material.  The 
chromatic  network  of  the  nucleus  may  be  either  dense,  or  so 
open  as  to  give,  even  after  staining,  the  appearance  of  a  lighter 


FIG.  45. — Axial  section  through  the  oocyte  of  the  Annulate,  Nereis.  After 
Lillie,  slightly  modified,  c,  cortical  protoplasmic  layer  (exoplasm) ;  n,  nucleus; 
no,  nucleolus;  o,  oil  vacuoles;  v,  vitelline  membrane;  y,  yolk  bodies. 

area  in  the  cytoplasm  (Figs.  43,  45),  often  known  as  the  germinal 
vesicle.  In  some  eggs  the  nucleus  is  in  the  process  of  division  at 
the  time  of  egg-deposition  (Figs.  42,  46);  this  condition  will 
be  explained  later.  The  actual  morphological  composition  of 
the  nucleus  is  one  of  the  chief  characteristics  of  the  egg, 
and  this  too  is  reserved  for  consideration  in  Chapter  IV. 
There  is  commonly  a  large  plasmosome  or  nucleolus  of  varying 
form  and  size  (Figs.  43,  45).  Centrosome  and  centrosphere  are 
typically  present  at  this  time,  though  often  minute  and  difficult 
to  observe;  later  these  structures  disappear  entirely.  Fre- 
quently the  cytoplasm  contains  unusual  bodies  termed  "yolk- 
nuclei."  The  term  yolk-nucleus  includes  organs  of  several 
different  types,  in  some  way  related  apparently  to  the  formation 


90  GENERAL  EMBRYOLOGY 

and  deposition  of  the  yolk 'and  certain  other  substances  within 
the  egg  cytoplasm.  Sometimes  the  cytoplasm  appears  homo- 
geneous, in  other  cases  it  shows  considerable  differentiation. 
Often  the  peripheral  layer  of  the  cytoplasm  is  more  vacuolated 
and  less  granular  than  the  central  portion;  the  former  is  then 
spoken  of  as  exoplasm  or  as  the  cortical  layer,  the  latter  as 
endoplasm  (Figs.  42,  43,  45,  46).  There  may  also  be  various 
materials  in  the  cytoplasm  which  have  been  laid  down  during 
the  formation  of  the  egg,  under  the  influence  of  the  nucleus,  or 
yolk-nucleus,  and  deposited  in  different  regions  (Figs.  42-48). 


FIG.  46. — Section  through  the  ovarian  egg  (oocyte)  of  Amphioxus.  After 
Sobotta.  X  525.  c,  vacuolated  cortical  layer  (exoplasm) ;  e,  endoplasm  con- 
taining deutoplasmic  bodies;  v,  vitelline  membrane;  /,  first  polar  body;  //, 
second  polar  spindle. 

The  extent  of  the  cytoplasmic  differentiation  varies  greatly 
in  eggs  of  different  species.  In  many  forms  it  can  hardly  be 
demonstrated  in  the  egg  at  the  time  it  is  fully  formed;  in  such 
eggs  this  differentiation  appears  later,  during  or  after  the  proc- 
esses of  fertilization,  or  even  still  later,  during  cleavage.  We 
shall  have  to  return  to  this  subj  ect  in  connection  with  the  sub- 
ject  of  cleavage,  after  we  have  described  the  fertilization  proc- 
ess. But  there  are  two  or  three  fundamental  aspects  of  this 
fact  that  should  be  mentioned  here.  There  are  quite  commonly 
three,  sometimes  more,  distinct  forms  of  cytoplasmic  material 
arranged  as  definite  regions  of  the  ovum,  occasionally  as  zones, 
or  layers,  or  as  localized  masses  (Figs.  42,  45).  These  may  be 
distinguished  by  the  more  or  less  vacuolated  character  of  the 


GERM  CELLS  AND  THEIR  FORMATION 


91 


protoplasm,  or  by  the  collection  of  various  pigments  and  dif- 

ferently colored  granules,  or  by  forms 

of  deutoplasmic  materials  other  than 

yolk,  or  in  various  other  ways.     The 

disposition  of  these  substances  usu- 

ally   expresses,    incompletely,  how- 

ever, an  underlying  organization  or 

morphology  of  the  egg  substance  as     f 

a  whole,  which  is  considered  a  fun- 

damental structure  of  the  egg  as  a 

specific     organism.     This    organiza- 

tion is  practically  always  polar,  i.e., 

disposed   symmetrically  with   refer- 

ence to  one  chief  axis  (Von  Baer), 

and  in  the  eggs  of  most  bilateral    d 

animals  examined,  it  is  bilateral  also 

(Roux,    Van    Beneden).      In   some 

way  this  morphology  of  the  egg  is 

related   to   the   morphology  of  the 

embryo  developed  from  the  egg,  and 

hence  is  called  its  promorphology. 

This  promorphology  is  better 
termed  organization,  for  it  is  not  only 
grossly  material,  but  also  dynamic, 
i.e.,  energetic,  depending  upon  chem- 
ical and  physical  arrangements  not 

often    visible    directly.      The    extent     drawing    of    a  median   section 
i  £   ,  -,  •  ,  •  through  the  fertilized  egg  of  the 

and  nature  of  this  organization  are    fly>  Musca.    From  Korscheit 

Often     Obscure,     but     this,     and    the     and  Heider,  after  Henking  and 

Blochmann.      ch,     chonon;    d, 
nuclear    Structure    of    the  OVUm,    are     flattened  dorsal  side  of  the  egg; 


FIG.  47. — Semidiagrammatic 


probably  its  most  important  char- 

acteristicS,  for   together  these    deter-  traded  through  the  micropyle; 

..    .          ,         ,  k,  cortical  layer;  m,  micropyle; 

mine  the  COUrse   Of    its    development  this  also  marks  the  anterior  end 

as    a    Specific    Creature.  °f   ^e   egg;  p,  egg  and  sperm 

r_                                            e  pronuclei  in  process  of  fusion  ;  r, 

Polarity    is   One  expression  Of  this  polar  bodies;  v,  ventral  side  of 

organization.     The   polarity   of  the 

fully  formed  ovum  is  related  to  the  polarity  of  the  egg  cell 


92  GENERAL  EMBRYOLOGY 

as  it  was  placed  in  the  epithelium  of  the  gonad,  the  chief 
egg  axis  corresponding  with  the  axis  passing  through  the 
attached  and  free  surfaces  of  the  epithelial  cell.  This  polarity 
apparently  determines  the  primary  position  of  the  egg  nucleus 
and  centrosomes,  and  thus  secondarily  determines  also  the 
arrangement  of  the  cytoplasmic  substances  which  develop 
through  the  interactions  between  the  nucleus  and  cytoplasm, 
processes  which  may  frequently  be  observed  during  the  growth 
period  of  the  ovum.  The  two  poles  of  the  egg  are  commonly 
unlike  (Figs.  42,  48),  so  that  we  distinguish  an  animal  and  a 
vegetal  pole,  corresponding  in  most  cases  with  the  originally 
free  and  attached  surfaces  respectively,  although  this  relation 
may  occasionally  be  reversed  (Echinoderms).  In  general 
the  animal  pole  is  that  toward  which  the  nucleus  is  eccentrically 
displaced,  and  nearer  which  the  centrosome,  or  similar  body,  is 
located;  it  is  the  more  protoplasmic,  and  therefore  the  more 
active  region  of  the  egg.  The  vegetal  pole  is  frequently  occu- 
pied largely  by  the  relatively  inert  food  substance,  the  materials 
in  general  related  with  the  vegetative  organs  of  the  developing 
embryo. 

As  regards  the  nature  of  the  organization,  or  promorphological 
relations,  of  the  egg,  two  views  have  been  taken  and  will  be  discussed 
in  Chapter  VII.  The  first  is  that  the  differentiated  substances  visible 
within  the  cytoplasm  are  genuinely  "  organ-forming  substances,"  or  at 
any  rate  tissue  forming,  in  their  potentiality.  Thus  they  represent  the 
organs  of  the  embryo  in  an  intracellular  form.  The  second  view  is  that 
these  substances  are  only  secondarily  related  to  the  real  morphology 
of  the  embryo,  and  that  both  embryonic  structure  and  the  differentiated 
substances  of  the  egg,  are  the  result  of  an  underlying,  invisible,  and  as 
yet  little-known  organization  of  the  ground  substance  of  the  egg  cyto- 
plasm. According  to  this  view  the  correspondence  between  the  "organ- 
forming  substances"  of  the  ovum,  and  the  organs  and  tissues  of  the 
embryo,  is  not  in  itself  direct,  but  results  from  their  common  relation  to 
the  primary  underlying  arrangement  or  organization  of  the  substance 
of  the  egg.  In  some  cases  the  arrangement  and  position  of  these  sub- 
stances may  be  considerably  altered  experimentally  without  disturbing 
the  normal  course  of  development.  It  should  be  added,  however,  that 
in  some  other  cases  such  a  disarrangement  does  effect  a  corresponding 
disarrangement  of  the  organs  or  tissues  of  the  embryo. 


GERM  CELLS  AND  THEIR  FORMATION  93 

In  addition  to  the  formative  substances  mentioned  above 
eggs  may  contain  varying  amounts  of  nutritive  substance  of 
many  different  kinds,  collectively  termed  yolk  or  deutoplasm. 
The  yolk  may  be  in  the  form  of  granules,  small  spherical  bodies, 
large  plates,  fluid  drops  of  various  sizes,  or  in  compact  masses 
(Figs.  45,  48).  These  substances  may  be  of  different  chemical 
compositions  and  staining  reactions  in  a  single  egg.  They 
may  be  formed  within  the  egg  by  its  own  activity,  or  they  may 
be  contributed  indirectly  by  cells  associated  with  the  egg  during 
its  formation.  The  arrangement  of  the  food  substances  in  the 
egg  has  an  important  bearing  upon  its  later  development,  espe- 
cially upon  the  form  of  its  cleavage  (Balfour).  Eggs  in  which 
the  yolk  is  distributed  quite  uniformly  through  the  cytoplasm, 
and  in  which  the  protoplasm  is  therefore  more  or  less  com- 
pletely intermingled  with  the  yolk  granules,  or  plates,  are 
termed  homolecithal  or  isolecithal  eggs.  Some  eggs  have  been 
described  as  aledthal,  i.e.,  without  yolk,  but  many  of  these 
have  been  found  really  to  contain  a  small  amount  of  quite 
uniformly  distributed  deutoplasm,  and  a  truly  alecithal  egg  is 
rarely  if  ever  found.  Eggs  of  some  species  among  nearly  all 
the  large  groups  are  of  this  homolecithal  type,  for  example, 
the  star-fish,  sea-urchin,  and  also  the  Mammals,  which  were 
formerly  thought  to  be  alecithal  (Fig.  43).  More  frequently 
the  yolk  and  cytoplasm  are  not  uniformly  mingled  but  are 
chiefly  accumulated  in  different  parts  of  the  cell.  Ordinarily 
these  materials  occupy  opposite  poles  of  the  egg  so  that  this 
retains  a  radial  or  rotatorial  symmetry;  the  yolk  is  accumulated 
toward  the  vegetative  pole,  the  protoplasm  toward  the  animal 
pole  (Fig.  48).  Such  eggs  are  termed  telokcithal.  They  show 
great  variation  in  the  relative  amount  of  yolk  contained.  On 
the  one  hand  it  is  often  difficult  to  distinguish  the  telolecithal 
egg  from  the  homolecithal  type,  for  the  tendency  toward  polar 
accumulation  of  the  yolk  may  be  very  slight.  The  egg  of 
Amphioxus  illustrates  such  a  transitional  condition.  At  the 
opposite  extreme  we  find  eggs  such  as  those  of  the  Reptiles  and 
Birds,  which  are  relatively  immense  cells,  in  which  it  is  difficult 
to  distinguish,  before  development  begins,  any  definite  region 


94 


GENERAL  EMBRYOLOGY 


which  is  entirely  free  from  yolk.  Between  these  extremes  all 
intermediate  conditions  are  found.  This  telolecithal  type  of 
egg  is  very  common  among  the  Invertebrates,  and  is  charac- 
teristic of  all  the  Craniata  except  the  true  Mammals.  Among 
the  Chordata  successive  stages  in  the  accumulation  of  yolk  are 
represented  by  Amphioxus,  Lampreys,  Ganoids,  Dipnoans, 
Amphibians,  Reptiles,  and  Birds.  In  the  last  two  groups  the 


d 


FIG.  48. — Egg  of  the  Teleost,  Fundulus  heteroclitus.  Total  view,  about  an 
hour  after  fertilization,  c,  chorion;  d,  protoplasmic  germ  disc  or  blastodisc; 
o,  oil  vacuoles;  p,  perivitelline  space;  v,  vitelline  membrane;  y,  yolk. 


protoplasm  is  extremely  limited  in  amount,  and  is  found  only 
as  a  small  disc  or  layer  on  the  surface  of  the  spherical  yolk- 
mass,  at  the  animal  pole.  A  third  and  less  common  arrange- 
ment of  yolk  is  that  seen  in  the  centrolecithal  eggs  of  many 
Arthropods,  chiefly  Insects.  Here  the  yolk  occupies  a  greater 
or  lesser  portion  of  the  center  of  the  egg  while  the  protoplasm 
forms  a  superficial  layer  all  around  it  (Figs.  47,  117,  11$). 


GERM  CELLS  AND  THEIR  FORMATION 


95 


The  various  constituent  materials  of  the  egg  differ,  often 
very  considerably,  in  density,  and  since  they  are  definitely 
disposed  with  reference  to  the  chief  egg  axis,  the  eggs  tend  to 
assume  a  definite  position  with  respect  to  gravity  when  they 
are  free  to  move.  Usually,  in  such  cases,  the  yolk  is  heavier 
than  the  protoplasm,  and  the  animal  pole  is  therefore  directed 
upward;  this  position  is  reversed  occasionally,  particularly 
when  the  deutoplasm  is  in  the  form  of  oil  drops,  e.g.,  Nereis  and 
most  Teleosts. 

In  some  forms  the  egg  cells  are  naked,  without  cell  coverings 
or  membranes,  as  in  many  Ccelenterates  and  some  Molluscs. 
Or  the  egg  may  be  naked  at  first,  but  soon  after  becoming  free 
from  the  parental  body  it  may  acquire  a  thin  membrane  over 
its  surface,  as  in  Echinoderms.  Ordinarily  the  egg  is  surrounded 
by  definite  membranes  of  varying  nature  and  origin.  The  pri- 
mary egg  membrane  is  the  vitelline  membrane, 
or  true  egg  membrane.  Typically  this  is  a 
thin  membrane  secreted  by  the  superficial 
protoplasm  of  the  egg  and  closely  applied  to 
its  surface  (Figs.  42,  45,  46,  47).  In  most 
cases  it  is  quite  structureless;  sometimes  it 
is  thicker  and  may  be  perforated  radially 
by  minute  canals  or  pores,  when  it  is  termed  FlG-  49.— Section 

,,  ,.  ~  .         n        ,          .     ,,.  through  the  egg  mem- 

the  zona  radiata.     Occasionally  the  vitelline    branes  of  the 
membrane  may  appear  double,  showing  an    mobranch- 


inner  zona  radiata  and  an  outer  structureless 
layer   (Figs.  49,   50).     In  such  cases  it   is 


(Ovarian  egg.)  From 
Ziegler,  after  Balf our. 
fe,  Follicular  epithe- 
lium;^, outer  portion 


possible  that   the  membrane  is  not  wholly    of  the  viteffine  m«n- 

.,    „.  J      brane     (zona     pellu- 

vitellme,  or  it  may  be  the  case  more  fre-    cida);  yk,  surface  of 
quently  that    the    pores    originally  passed    %™  ™f:  f£  ™£ 
completely  through  the  membrane  and  dis-    line.  membrane  (zona 
appeared  from  the  outer  portion  of  it  upon 
contact  with  an  external  medium. 

The  vitelline  membrane  may  envelop  the  egg  completely,  or 
there  may  be  left  a  minute,  funnel-shaped  perforation  through 
it  at  the  point  where  the  egg  was  attached  or  otherwise  espe- 
cially related  to  the  epithelium  of  the  ovary.  This  aperture 


96 


GENERAL  EMBRYOLOGY 


is  the  micropyle  (Fig.  50),  and,  as  we  shall  see,  this  sometimes 
affords  an  aperture  for  the  entrance  of  the  sperm  cell. 

A  secondary  membrane  called  the  chorion  often  surrounds  the 
egg  outside  the  vitelline  (Figs.  42-50).  This  is  a  secretion 
formed  while  the  egg  is  still  contained  within  the  ovary,  from 
the  cells  there  surrounding  the  egg,  and  its  presence  depends 
upon  the  arrangement  of  these  ovarian  cells  in  the  form  of  a 
definite  layer  or  epithelium  surrounding  the  egg,  termed  the 


pb 


B 


FIG.  50. — A.  Animal  pole  of  the  egg  of  the  Cephalopod,  Argonauta.  From 
Wilson,  "Cell,"  after  Ussow.  Surrounding  the  egg  is  the  chorionic  membrane 
perforated  by  the  funnel-shaped  micropyle,  m.  Beneath  the  micropyle  lies  the 
egg  nucleus  in  the  cortical  protoplasmic  layer,  p.b,  polar  bodies.  B,  C.  Sec- 
tions through  the  egg  membranes  and  micropyle  of  the  egg  of  the  Teleosts  Esox 
(B)  (ovarian  egg)  and  Pygosteus  (C)  (ovarian  egg,  0.4  mm.  in  diameter).  After 
Eigenmann.  X  375.  ez,  zona  radiata  externa;  /,  egg  follicle;  iz,  zona  radiata 
interha;  m,  micropylar  cell;  p,  egg  protoplasm;  pp,  protoplasmic  processes; 
y,  yolk  in  egg;  z,  zona  radiata. 

follicle.  The  chorion  may  be  a  thin  flexible  membrane,  or  a 
tough  resistant  shell,  as  in  Insects  and  Teleosts.  Penetrating 
the  chorion  there  is  nearly  always  a  continuation  of  the  micro- 
pyle. The  formation  of  this  results  from  the  fact  that  one  of 
the  cells  of  the  follicle  usually  acquires  a  very  intimate  relation 
with  the  ovum,  through  a  fine  pseudopodial  process,  »o  that 
at  this  point  no  membranes  are  laid  down  (Fig.  50).  When  the 


GERM  CELLS  AND  THEIR  FORMATION  97 

egg  is  fully  formed  an'd  leaves  the  follicle,  this  process  is  with- 
drawn, leaving  a  funnel-shaped  canal.  In  a  few  instances 
(some  Insects)  there  are  several  micropylar  perforations 
through  the  egg  membranes.  Such  openings  are  to  be  regarded 
as  specializations  of  the  minute  canals,  mentioned  above,  which 
give  the  appearance  of  the  zona  radiata  to  the  membranes. 

Finally  there  is  a  great  variety  of  tertiary  membranes  formed 
by  the  walls  of  the  oviducts,  or  by  special  glands  in  connection 
with  the  reproductive  system.  These  are  applied  outside  the 
chorion,  or  if  this  is  absent,  directly  upon  the  vitelline  mem- 
brane. These  envelopes  may  be  of  slime  or  jelly  of  an  albu- 
minous character,  fibrous,  or  shelly  coverings  of  chitin,  lime,  or 
other  substance.  In  forms  depositing  the  eggs  in  the  water  this 
is  sometimes  a  thick  jelly,  holding  the  eggs  together  in  strings 
or  masses,  or  serving  to  attach  them,  either  singly  or  in  masses 
to  plants,  sticks,  or  other  solid  objects  (e.g.,  Amphibia).  These 
tertiary  membranes  serve  also,  in  special  instances,  as  protec- 
tion against  drying,  temperature  changes,  pressure,  or  mechani- 
cal injury,  and  against  the  attacks  of  food-seeking  organisms, 
or  infection  by  bacteria  or  other  parasitic  organisms.  Fre- 
quently they  are  nutritive  in  character,  as  the  albumin  or 
"  white"  of  the  birds'  eggs  or  the  dense  oily  substance  surround- 
ing the  eggs  of  the  snails. 

Eggs  may  possess  none  or  all  three  of  these  classes  of  mem- 
branes ;  sometimes  only  primary  and  secondary,  or  primary  and 
tertiary  membranes  are  present.  This  usually  depends  upon 
the  nature  of  the  egg-laying  habits,  method  and  duration  of 
development,  and  various  other  conditions. 

The  spermatozoa,  when  fully  formed,  bear  little  resemblance 
to  ordinary  cells,  yet  their  individual  history  clearly  shows  them 
to  be  such.  In  a  few  forms,  chiefly  among  the  Crustacea,  the 
sperms  do  resemble  ordinary  cells,  and  are  often  provided  with 
long  radiating  processes,  sometimes,  though  rarely,  pseudopo- 
dial.  But  by  far  the  most  common  form  is  that  known  as  the 
flagellate  spermatozoon,  found  in  all  groups  of  animals  from  Pro- 
tozoa to  man.  These  are  minute  thread-like  cells  in  which  three 
general  regions  can  usually  be  made  out  (Fig.  51).  One  end  is 


98 


GENERAL  EMBRYOLOGY 


n 
ac 
pc 

=  s 

mi 

a 


—  af 


FIG.  51.— Flagellate  sper- 
matozoon. A,  B.  Two  views 
of  human  sperm  cell.  After 
Retzius.  X  2000.  C.  Dia- 
gram of  the  structure  of  a 
generalized  type  of  flagellate 
spermatozoon.  After 
Meves.  a,  annulus;  ac, 
anterior  centrosome;  af,  ax- 
ial filament;  c,  centrosomes 
(end  knobs) ;  e,  protoplasmic 
envelope;  h,  head;  m,  middle 
piece;  mi,  mitochondria;  n, 
nucleus ;  ne,  neck ;  p,  perf  ora- 
torium  (acrosome) ;  pc,  pos- 
terior centrosome;  s,  spiral 
filament;  /,  tail  piece;  if, 
terminal  filament. 


enlarged  forming  the  so-called  head. 
This  is  in  reality  chiefly  made  up  of  the 
nucleus  of  the  cell,  and  it  stains  densely 
with  all  nuclear  dyes.  The  chromatin 
of  the  nucleus  is  solidly  packed,  and 
though  the  head  of  the  sperm  is  much 
smaller  than  the  egg  nucleus,  the  two 
contain  practically,  perhaps  precisely, 
equal  amounts  of  chromatin.  Just 
behind  the  head  is  a  smaller  middle 
piece  which  is  the  chief  cytoplasmic 
portion  of  the  cell;  the  cytoplasm  is 
really  continued  as  a  very  thin  envelope 
over  the  head,  at  the  anterior  end  of 
which  it  is  usually  produced  as  a  sharp- 
ened perforating  or  attaching  organ 
called  the  acrosome  or  perforatorium. 
In  some  spermatozoa  (e.g.,  some  Mam- 
malia) the  head  is  connected  with  the 
middle  piece  by  an  intermediate  section 
called  the  neck  (Fig.  51).  The  middle 
piece  is  quite  highly  differentiated.  It 
contains  the  centrosomal  structures  of 
the  spermatozoon,  and  its  center  is  oc- 
cupied by  the  proximal  portion  of  a 
kinoplasmic  structure,  the  axial  fila- 
ment. Surrounding  this  are  frequently 
one  or  more  differentiated  layers,  and 
often  a  spirally  wound  thread;  the 
whole  is  covered  with  a  dense  outer 
sheath.  In  some  instances  (toad)  the 
centrosome  is  said  to  be  included  in  the 
region  of  the  head  piece.  Extending 
posteriorly  from  the  middle  piece  is  a 
long  flagellum  or  tail,  in  some  species 
flattened  and  provided  with  a  fin-like 
undulatory  membrane  (Fig.  52,  L). 


GERM  CELLS  AND  THEIR  FORMATION 


99 


N  > 


FIG.  52. — Various  types  of  spermatozoa.  A,  B.  The  Teleost,  Leucisciis 
(Ballowitz).  C,  D.  The  birds,  Phyllopneuste  and  Tadorna  (Ballowitz).  E,  F. 
Two  forms  of  the  sperm  of  the  snail,  Paludina  (von  Brunn).  G.  The  Nematode, 
Ascaris  (Van  Beneden).  H.  The  Annulate,  Myzostoma  (Wheeler).  /.  The 
bat,  Vesperugo  (Ballowitz)^  tJt.  The  opossum, t  Didelphys  (Wilson),  K.  The 
rat  (Wilson).  L.  The  Urodete,' A-mptou/no -(McGregor).  "  'M,  Tfre  Crustacean, 


100  GENERAL  EMBRYOLOGY 

Through  the  middle  of  the  tail  is  an  axial  filament  connect- 
ing with  the  centrosome  of  the  middle  piece,  a  relation 
which  is  very  common  in  flagellate  or  ciliate  structures. 
Proximally  the  axial  filament  passes  through  a  ring-like 
structure,  the  annulus,  in  the  end  of  the  middle  piece. 
Distally  the  filament  may  continue  beyond  the  cytoplasmic 
envelope  of  the  tail  as  a  terminal  filament  or  end  piece.  There 
is  the  greatest  variety  in  details  of  form  of  the  sperm,  affecting 
chiefly  the  form  of  the  head  and  acrosome,  length  of  tail,  and 
total  size;  a  few  of  the  more  common  or  more  striking  forms 
are  illustrated  in  Fig.  52.  (In  cases  where  a  neck  region  is 
distinguished,  the  middle  piece  is  often  regarded  as  the  proximal 
part  of  the  tail.) 

The  smallest  spermatozoa  are  found  in  Amphioxus  and 
are  only  0.016-0.020  mm.  (16-20  micro}  in  length;  the  largest 
are  found  in  some  of  the  Amphibia  where  in  Salamandra  they 
are  about  0.7  mm.  (700  micro)  in  length,  and  in  Discoglossus 
2.0  mm.  (2000  micro),  the  maximum  length  known.  The  sperm 
cells  of  the  Crustacean  Cypris,  are  also  of  this  gigantic  size 
(2  mm.).  The  spermatozoa  of  most  of  the  Vertebrates  are 
25-75  micro  in  length.  The  human  spermatozoon  (Fig.  51) 
represents  about  the  average  size;  -the  dimensions  of  this  are 
(Krause) :  total  length,  52-62  micro  (1/400-1/500  inch) ;  length 
of  head,  about  4.5  micro  (between  1/5000  and  1/6000  inch); 
length  of  middle  piece,  about  6  micro  (about  1  /4200  inch) ; 
length  of  tail  piece,  41-52  micro  (1/500  to  1/600  inch) ;  width 
of  head  2-3  micro  (1/12000  to  1/8000  inch);  thickness  of  head, 
(this  is  one  of  the  few  instances  where  the  head  is  flattened), 
about  1  micron  (1/25000  inch). 

The  number  of  sperm  formed  by  a  single  individual  is  very 
large  in  most  organisms  and  can  be  only  roughly  estimated. 
It  has  been  computed  (Lode)  that  the  total  number  formed  in 
man  may  average  about  three  hundred  and  forty  billion,  or 


Ethusa  (Grobben).  N.  The  Crustacean,  Inachus  (Grobben).  O.  The  Crusta- 
cean Sida  (Weismann) .  P.  The  Crustacean,  Bythotrephes  (Weismann).  A;,  end 
knob;  m,  middle  piece;  w,rnuqleus;  p,  per/ oral oriurn;  u,  undulatory  membrane. 
Not  drawn  «fo  same  Male.  A-F,  'l-K,vfrjm 


GERM  CELLS  AND  THEIR  FORMATION          101 

approximately  eight  hundred  and  fifty  million  sperm  for  each 
one  of  the  four  hundred  ova  matured  during  the  reproductive 
period  of  the  female.  The  volume  of  the  human  sperm  is 
roughly  only  about  1/195000  that  of  the  egg  (egg  =0.25  mm.  in 
diameter).  The  sperm  of  the  sea-urchin  contains  only  about 
1  -400000  to  1/500000  the  material  in  the  egg  (Wilson). 

In  several  forms,  both  Vertebrate  and  Invertebrate,  atypical 
"giant"  spermatozoa  are  occasionally  found  (Fig.  52,  E).  In 
most  instances  these  are  abnormalities  resulting  from  some 
deviation  from  the  usual  course  of  events  during  sperm  for- 
mation. But  in  a  few  instances  (Euschistus)  a  dimorphism  of 
the  sperm  seems  entirely  normal  (Montgomery).  In  such  cases 
the  larger  sperm  heads  contain  the  normal  amount  of  chro- 
matin,  but  an  excessive  amount  of  linin  and  karyolymph. 

In  marked  contrast  to  the  egg,  the  sperm  cells  are  in  very 
active  movement  on  account  of  the  rapid  vibration  of  the  tail. 
They  are  always  contained  within  a  fluid  medium,  the  seminal 
fluid,  which  is  either  the  fluid  of  the  cavities  of  the  body,  or  a 
special  secretion  of  certain  glands  in  connection  with  the 
reproductive  system.  In  the  latter  case  this  fluid  is  equivalent 
to  a  tertiary  egg  envelope,  and  like  this  is  sometimes  of  nutritive 
value  to  the  germ  cells. 

The  form  differences  between  ova  and  sperm  give  a  nice 
illustration  of  the  modification  of  structure  accompanying  a 
physiological  division  of  labor,  which  is  so  well  marked  here. 
Both  cells  contain  equal  amounts  of  nuclear  substance,  but  the 
ovum  possesses  in  addition  a  large  amount  of  cytoplasm,  and 
often  a  much  larger  amount  of  food  substance,  while  the  sperm 
contains  an  amount  of  cytoplasm  which  represents  practically 
an  irreducible  minimum,  and  no  deutoplasmic  material  what- 
ever. The  egg,  therefore,  provides  practically  the  whole  of 
the  extranuclear  substance  of  the  developing  organism.  At 
the  same  time  the  ovum  is  a  passive,  non-motile  structure. 
The  spermatozoa,  on  the  other  hand,  are  not  only  extremely 
motile,  but  they  are  produced  in  very  large  numbers,  conditions 
correlated  with  their  function  of  finding  the  inactive  ova  and  of 
ensuring  the  initial  stimulus  to  activity  (development)  of  the 


102  GENERAL  EMBRYOLOGY 

passive  egg  material.  This  differentiation  affords  the  material 
basis  for  development  while  at  the  same  time  it  ensures  the 
fertilization  of  practically  every  egg  produced,  though  the  eggs 
and  sperm  may  be  shed  freely  into  the  water  some  distance  apart, 
a  distance  often  very  great  as  compared  with  the  size  of  the 
cells.  The  relatively  very  large  amount  of  cytoplasm  in  the 
egg,  and  small  amount  in  the  sperm,  constitute  the  most 
marked  exceptions  to  the  nucleo-cytoplasmic  (kern-plasma) 
relation,  mentioned  in  the  preceding  chapter;  these  conditions 
are  entirely  special  and  are  to  be  regarded  as  adaptations  to 
the  very  unusual  functions  of  these  cells. 

The  details  concerning  the  form  and  number  of  the  sperm  and  egg 
cells,  the  amount  of  yolk  in  the  eggs,  the  character  of  their  membranes, 
etc.,  are  significant  only  from  the  viewpoint  of  adaptedness  to  the  con- 
ditions under  which  they  must  function.  This  adaptedness  of  the  re- 
productive phenomena  toward  ensuring  the  final  bringing  to  maturity 
of  a  number  of  organisms  sufficient  to  maintain  the  specific  group 
in  undiminished  numbers  is  a  general  biological  topic  of  especial  interest. 
In  strictness  this  lies  outside  our  province,  but  to  omit  entirely 
any  reference  to  this  subject,  leaves  without  significance  many  of 
the  details  of  structure  and  behavior  mentioned  in  the  preceding 
paragraphs.  We  may  therefore  suggest  briefly  a  few  of  these  relations, 
not  only  as  regards  the  germ  cells,  but  also  the  general  processes  of 
spawning,  etc.,  which  are  all  concerned  in  finally  bringing  together  an 
ovum  and  a  spermatozoon. 

All  these  varied,  and  often  complex,  phenomena  of  habit  and  morpho- 
logical specialization  of  the  reproductive  cells  are  correlated  with  the 
special  conditions  of  life  which  affect  the  chances  that  a  single  egg  shall 
finally  become  a  mature  organism.  They  are  conveniently  grouped 
under  three  chief  heads:  (a)  the  ensurance  of  mating,  (b)  the  ensurance 
of  the  actual  meeting  and  fusion  of  the  germ  cells,  (c)  the  chances  of  death 
before  maturity,  involving  such  factors  as  abundance  of  food,  enemies, 
adverse  conditions  in  the  inorganic  surroundings,  necessity  for  reaching 
special  conditions  of  development,  food,  etc.,  duration  of  the  period  of 
development,  and  the  like. 

A  few  forms,  especially  in  the  warmer  climates,  appear  to  breed  quite 
continuously  throughout  the  year  (many  Coelenterates,  Mollusca,  etc.), 
but  commonly  the  germ  cells  are  produced  at  regular  periods,  which  may 
have  a  duration  of  only  a  few  days  or  hours,  or  they  may  extend  over 
several  months.  Breeding  or  spawning  periods  are  nearly  always  sea- 
sonal and  usually  annual,  but  a  few  forms,  particularly  the  Mammals, 


GERM  CELLS  AND  THEIR  FORMATION          103 

breed  or  spawn  at  shorter  intervals.  In  many  Mammals  it  is  true  that 
there  is  only  a  single  annual  breeding  season  or  period  of  cestrus;  this 
condition,  known  as  moncestrous,  is  characteristic  of  most  Carnivora 
and  is  found  also  in  the  Chiroptera  and  Marsupials.  Others,  however, 
are  polycestrous,  and  exhibit  two  or  three  annual  breeding  seasons 
(Insectivors),  and  in  still  others  the  period  of  oestrus  may  occur  at  inter- 
vals of  a  few  weeks  (man),  or  it  may  be  quite  continuous,  as  in  most 
Rodents  and  some  Carnivors.  (See  Marshall,  Physiology  of  Reproduc- 
tion, 1910.) 

Among  the  higher  animals  the  breeding  season  is  often  preceded  by  a 
"nuptial  season"  during  which,  especially  among  the  males,  there  may 
develop  various  special  morphological  «and  physiological  peculiarities. 
The  Fishes,  Birds,  and  Mammals  exhibit  the  frequent  development  of 
special  external  markings  or  colorings,  special  secretions,  and  unusual 
modes  of  behavior.  Both  these  and  the  breeding  habits  proper,  are  to 
be  regarded  as  responses  to  stimuli,  frequently  climatic  in  origin, 
resulting  from  changes  in  temperature,  light,  moisture,  food  characters, 
etc.  These  phenomena  are  commonly  regarded  as  indications  of  an 
increased  metabolism  that  affects  not  only  the  organs  of  reproduction, 
but  secondarily  the  whole  body. 

In  a  few  rare  instances,  chiefly  among  the  segmented  worms,  the  an- 
nual spawning  season  is  very  definitely  fixed  and  varies  within  limits 
of  only  a  few  calendar  days.  More  usually  the  time  of  spawning  is 
subject  to  wide  variation  and  is  dependent  upon  temperature  and  other 
seasonal  conditions.  The  species  of  the  Palolo  worm  afford  one  of  the 
best  marked  instances  of  a  fixed  spawning  season.  It  is  not  quite  as 
regular  and  limited  as  tradition  would  have  it,  but  in  the  Tortugas, 
most  of  the  individuals  of  the  Atlantic  Palolo  (Eunice  fucata)  swarm  and 
spawn  during  one  or  two  mornings  which  fall  within  three  days  of  the 
moon's  last  quarter  between  the  latter  part  of  June  and  the  end  of  July 
(Mayer).  The  Pacific  species  (E.  viridis)  spawns  similarly  on  and  near 
the  last  quarter  in  October  and  November.  Somewhat  similar  relations 
have  been  determined  for  other  Annulata,  such  as  Amphitrite  (Scott) 
and  Ceratocephale.  The  determining  factor  in  these  and  similar  cases 
seems  to  be  the  character  of  the  tides,  combined  with  factors  of  tempera- 
ture and  light.  Other  organisms  spawn  at  a  definite  time  of  day, 
individuals  coming  to  maturity  at  any  time  during  a  longer  breeding 
season.  Thus  Amphioxus  and  some  Hydroids  spawn  only  about  sun 
down  or  shortly  thereafter. 

During  the  intervals  between  the  breeding  periods  the  formation  of 
the  germ  cells  may  almost  or  quite  cease,  to  recommence  shortly  prior 
to  the  next  period.  In  some  creatures,  however,  the  eggs  are  formed 
continuously  and  are  stored  in  secondary  reproductive  cavities  pending 
the  time  of  their  production.  This  is  more  likely  to  be  the  case  in  sperm 


104  GENERAL  EMBRYOLOGY 

production.  But  in  all  such  cases  the  rate  of  formation  of  the  germ 
cells  is  rhythmic,  increasing  just  before  the  breeding  period. 

The  sperm  cells  are  always  passed  outside  the  body  of  the  organism 
forming  them  (save  in  self-fertilizing  hermaphrodites) ;  the  eggs  may 
or  may  not  be  thus  extruded.  Animals  used  to  be  described  (even  clas- 
sified) as  "oviparous"  or  "  viviparous,"  according  to  whether  the  female 
extruded  undeveloped  eggs  or  living  "  young,"  but  these  terms  have  now 
lost  all  precise  meaning,  for  in  any  case  eggs  are  formed,  and  in  different 
species  the  developing  organisms  may  leave  the  body  or  reproductive 
cavity  of  the  parent  at  almost  any  stage. 

The  unfertilized  eggs  may  be  simply  thrown  outside  the  body  of  the 
female,  as  in  most  aquatic  animals,  the  sperm  being  thrown  out  at  the 
same  time  and  in  approximately  the  same  place;  in  such  cases  fertiliza- 
tion is  ensured  chiefly  by  the  production  of  immense  numbers  of  sperma- 
tozoa. Such  a  process  is  very  common  among  the  Sponges,  Coelen- 
terates,  Echinoderms,  Annulata,  Mollusca,  Fishes,  and  many  Amphibia. 
Eggs  thus  thrown  off  into  the  water  may  float  at  or  near  the  surface,  as 
pelagic  eggs,  or  they  may  sink  to  the  bottom  among  the  debris  (demersal) . 
Or  the  extruded  eggs  may  be  deposited  with  reference  to  definite  and 
often  very  special  conditions  affording,  to  the  new  organisms,  protec- 
tion, food,  etc.  Among  land  animals  which  deposit  the  eggs  outside  the 
body,  these  are  usually  very  definitely  placed  with  reference  to  such 
conditions ;  the  Insects  afford  a  great  variety  of  excellent  illustrations  of 
relations  of  this  kind.  In  some  cases,  among  both  aquatic  and  terres- 
trial forms,  definite  " nests"  are  constructed  in  which  the  eggs  are 
deposited,  and  where  the  newly  hatched  organisms  may  remain  for 
some  time.  The  eggs  and  young  then  may  or  may  not  be  guarded 
or  fed,  by  either  or  both  of  the  parents.  The  nests  may  vary  from 
simple  depressions  or  pockets  in  the  mud  or  sand,  like  those  of  many 
fresh  water  Fishes,  to  the  structures  of  very  complex  architecture  of 
many  Birds. 

Among  the  forms  which  do  not  liberate  their  eggs  at  an  early  stage 
in  their  development,  there  is  a  great  variety  of  habit.  In  some  Crus- 
tacea and  Amphibia,  for  example,  the  eggs  are  first  extruded,  but  are 
immediately  placed  upon  the  surface  of  the  body,  of  either  the  male 
or  female  parent,  and  develop  there.  Or  they  may  become  embedded 
in  the  skin  (many  Amphibia)  or  may  be  deposited  in  some  cavity  not 
primarily  a  reproductive  cavity,  such  as  the  pharyngeal  cavity,  in  some 
of  the  Siluroid  and  Cichlid  Fishes.  In  one  of  the  Cyprinoid  Fishes 
(Rhodeus)  the  eggs  are  placed  in  the  mantle  cavity  of  a  clam,  where 
they  are  fertilized  and  develop  on  the  gills.  In  most  cases  where  the 
eggs  are  retained  in  "brood  cavities,"  these  are  modified  portions  of 
some  part  of  the  reproductive  system  proper ;  here  the  eggs  may  remain 
until  a  comparatively  late  period  in  their  development.  In  such  cases 


GERM  CELLS  AND  THEIR  FORMATION 


105 


fertilization  must  be  internal,  and  the  sperm  are  then  definitely  intro- 
duced into  some  reproductive  cavity  of  the  female. 

An  interesting  series  of  relations  may  be  traced  illustrating  the  grad- 
ual increase  in  the  certainty  with  which  fertilization  shall  be  accom- 


B 


FIG.  53. — Forms  of  spermatophores.  A,  B,  C.  The  Insects,  Loricera,  Locusta, 
and  an  Ichneumonid.  From  Korschelt  and  Heider,  after  Gilson.  D,  E.  The 
Urodeles,  Amblystoma  and  Diemyctylus.  From  Bertram  Smith.  X  3.  F.  Peri- 
patus  edwardsii  (incompletely  developed).  After  von  Kennel  (Korschelt  and 
Heider). 

plished,  whether  it  be  external  or  strictly  internal.  First,  among  many 
of  the  Crustacea,  Annulata,  Gasteropods,  Cephalopods,  and  Amphibia, 
there  is  a  process  of  pseudo-copulation  or  amplexus,  where,  sometimes 


106  GENERAL  EMBRYOLOGY 

after  a  complicated  "courtship"  (Urodeles,  Jordan),  the  sperm  are 
received  by  the  female  in  or  near  the  reproductive  cavities  or  openings. 
Usually  in  such  instances  the  sperm  are  not  scattered  freely,  but  are 
contained  within  definite  packets  or  cases  called  spermatophores  (Fig. 
53).  In  the  Urodeles  these  are  simple  masses  of  spermatozoa  enclosed 
in  a  thin  envelope;  they  are  discharged  by  the  male  and  then  picked  up 
by  the  cloacal  margins  of  the  female  and  stored  until  the  eggs  are  ready 
to  be  fertilized.  In  the  amplexus  of  the  earthworm  and  many  Gastero- 
pods,  there  is  a  mutual  exchange  of  spermatozoa  between  two  herma- 
phroditic individuals,  the  sperm  being  received  into  storage  cavities 
and  retained  until  the  eggs  are  deposited.  Such  a  receipt  of  sperm  or 
sperm  packets  into  storage  cavities  is  quite  common,  and  the  sperm 
may  in  these  cases  remain  alive  for  long  periods,  even  for  three  or  four 
years  (honey  bee,  some  snails).  The  spermatophores  are  sometimes 
very  elaborate  affairs  containing  a  complex  mechanism  arranged  so  as 
to  discharge  the  sperm  just  at  the  time  the  female  is  depositing  the  eggs 
(Fig.  53,  C).  Among  the  more  complex  are  those  formed  by  the  male 


FIG.    54. — The   Trematode,    Bilharzia   hcematobia.     Two   individuals   living   in 
copula.     After  Fritsch.     X  14.     o,  ova  in  oviduct.     d\  male;  $,  female. 

squid  (Loligo)  and  transferred  to  the  buccal  membrane  of  the  female, 
where  they  remain  attached  pending  the  time  of  spawning  (Drew). 

Among  forms  where  internal  fertilization  is  the  rule,  this  is  more 
frequently  the  result  of  a  definite  act  of  copulation,  by  which  the  sperm 
are  transferred  directly  to  a  reproductive  cavity  of  the  female  through  a 
male  intromittent  organ.  This  occurs  in  most  Insects,  many  Turbellaria, 
Crustacea,  Molluscs,  and  in  most  of  the  higher  Vertebrates.  This  gen- 
eral relation,  carried  to  an  extreme  may  result  in  symbiosis,  or  even  in 
the  parasitic  character  of  the  male  upon  or  within  the  body  of  the  female. 
Thus  in  some  of  the  Cirripedia,  degenerate  "  complemental "  males  are 
found  living  semiparasitically  within  the  body  of  the  female.  Several 
of  the  Trematoda  live  in  pairs  within  a  single  cyst.  In  Bilharzia  (Tre- 
matoda)  the  female  lives  permanently  in  a  groove  on  the  body  of  the 


GERM  CELLS  AND  THEIR  FORMATION          107 

male  (Fritsch)  (Fig.  54).  In  Diplozoon  (Trematoda)  two  hermaphrodite 
individuals,  at  first  entirely  separate,  become  permanently  fused  so  that 
the  openings  of  the  reproductive  ducts  are  in  apposition.  In  Syngamus 
(Xemathelminthes)  also,  the  male  lives  permanently  attached  to  the 
female.  The  climax  of  this  relation  is  afforded  by  Trichosomum  (Nema- 
thelminthes)  where  several  (2-5)  dwarfed  males  live  within  the  uterus 
of  the  female  (Leuckart) .  Or  it  might  be  said  that  the  climax  is  to  be 
seen  in  the  cases  of  self-fertilizing  hermaphrodites;  these  are  usually, 
internal  parasites,  where  the  chances  of  the ,  meeting  of  males  and 
females  would  be  practically  nil  (e.g.,  many  of  the  Trematodes  and 
Cestodes). 

The  retention  of  the  eggs  during  their  development,  within  a  brood 
cavity  is  primarily  a  protective  arrangement,  but  it  often  leads  to  the 
establishment  of  an  organic  nutritive  relation  between  the  embryos  and 
the  wall  of  the  cavity.  This  is  the  case  in  some  Tunicates,  Elasmo- 
branchs,  etc.,  and  of  course  it  reaches  its  climax  in  the  intimate  and 
extensive  organic  relation  between  embryo  and  oviducal  (uterine)  wall 
among  the  Eutherian  Mammalia. 

The  number  of  eggs  produced  during  each  reproductive  or  spawning 
period  varies  enormously,  and  is  related  to  a  variety  of  conditions  of 
development.  In  general  the  number  of  eggs  is  larger  when  there  are 
few  or  no  other  means  of  ensuring  the  complete  development  of  a 
number  of  organisms  sufficient  to  maintain  the  species  numerically. 
Any  structure  of  the  egg  or  habit  of  deposition,  adapted  to  ensure 
development,  is  likely  to  be  associated  with  a  reduction  in  the  number 
of  eggs  formed.  The  number  is  largest  among  forms  which  discharge 
the  eggs  at  random  or  where  they  are  subject  to  unfavorable  external 
conditions,  to  liability  to  the  attacks  of  parasites,  or  use  as  food  by  other 
organisms.  For  example,  the  marine  fishes  produce  very  large  numbers 
of  pelagic  ova,  the  codfish  is  said  to  form  8  to  10  millions  in  one  season ; 
and  in  a  species  of  sea-urchin  (Echinus)  a  single  female  is  said  to  have 
discharged,  in  a  single  season,  as  many  as  20  million  ova.  Where  very 
special  conditions  of  development  are  essential,  as  in  the  complicated 
life  histories  of  many  internally  parasitic  worms,  the  number  of  eggs  is 
very  large  and  fertilization  is  ensured  by  hermaphroditism. 

The  number  of  eggs  is  smallest  where  they  develop  within  a  brood 
cavity,  or  where  some  degree  of  parental  care  is  exercised.  In  a  few 
Mammals  and  Birds  only  a  single  egg  is  formed  at  each  breeding  period, 
and  in  these  groups  the  number  rarely  exceeds  eight  or  ten.  Further 
the  number  of  eggs  produced,  in  general  varies  inversely  with  the 
amount  of  food  substance  contained,  or  with  the  chances  of  the  young 
.finding  food  for  themselves  by  the  time  they  become  free  living. 

The  number  of  sperm  cells  formed  is  always  larger  than  the  number 
of  eggs,  and  often  reaches  many  millions.  The  number  is  likely  to  be 


108  GENERAL  EMBRYOLOGY 

smaller  among  forms  in  which  the  sperm  are  directly  or  indirectly  intro- 
duced into  the  reproductive  cavities  of  the  female.  Some  of  the  Crus- 
tacea afford  interesting  illustrations  of  this.  In  some  Ostracods  only  a 
few  hundred  very  active  spermatozoa  are  formed;  these  are  inserted 
directly  into  the  seminal  receptacles  of  the  female.  They  are  most 
remarkable  for  their  size — 2  mm.  in  length  in  a  few,  or  more  than  twice 
as  long  as  the  body  of  the  male.  In  Daphnia  the  number  of  sperm  may 
«be  only  twenty,  or  even  less,  six  to  eight  in  some  species.  These  too  are 
liberated  directly  into  the  brood  cavity  of  the  female,  which  forms  only 
two  eggs  at  a  time;  these  sperm  are  very  adherent,  and  are  said  to  be 
somewhat  amoeboid.  Indeed  the  spermatozoa  of  many  of  the  Crustacea 
are  unusually  interesting  on  account  of  their  atypical  form  and  behavior 
(Fig.  52).  Some  (Bythotrephes)  are  quite  like  ova,  large  (0.1  mm.), 
rounded,  and  quiescent,  depending  upon  a  peculiar  viscid  or  adherent 
quality  for  their  likelihood  of  attachment  to  the  egg.  Others  (some 
Decapods)  have  a  number  of  stiff  radiating  processes  which  seem  to 
function  by  catching  in  the  hair-like  bristles  surrounding  the  openings 
of  the  oviducts,  where  they  are  placed  in  amplexus. 

The  amount  of  food  yolk  contained  in  the  egg  is  related  also  to  the 
duration  of  the  embryonic  period  of  development,  or  to  the  rate  of 
development,  a  prolonged  embryonic  life  requiring  an  abundant  supply 
of  food  materials,  and  an  unusually  rapid  rate  of  development  depending 
upon  a  supply  of  easily  assimilable  nutritive  substance.  Such  a  relation 
is  illustrated  by  the  difference  between  summer  and  winter  eggs,  formed 
by  Rotifers,  and  many  Insects  and  Crustacea;  the  winter  eggs,  subject 
to  unfavorable  conditions  and  passing  a  longer  period  in  development, 
contain  considerably  more  yolk,  and  are  covered  with  much  tougher 
and  more  resistant  membranes,  than  the  summer  eggs  which  develop 
rapidly  and  under  favorable  surroundings,  indeed  often  within  the  brood 
cavity  of  the  female.  Thus  in  Daphnia  the  small  summer  eggs  are 
formed  by  only  three  nurse  cells  (see  below) ,  while  the  large  winter  eggs 
are  supplied  with  food  by  forty  or  more  nurse  cells.  When  the  developing 
embryo  acquires  special  nutritive  relations  with  the  parental  tissues, 
the  eggs  are  of  course  practically  yolkless. 

The  provision  of  egg  membranes  is  associated  not  only  with  a  reduction 
in  the  number  of  eggs  formed,  but  also  with  the  duration  of  the  em- 
bryonic period,  liability  to  unfavorable  external  conditions,  prevalence 
of  food-seeking  enemies,  etc.  The  membranes  which  are  functional 
under  such  conditions  are  of  the  secondary  and  tertiary  classes  described 
above.  The  nature  of  these  varies  from  the  common  thin  fibrous 
coverings,  to  tough  and  impervious  membranes  capable  of  resisting 
extreme  dryness,  or  the  leathery  or  calcareous  "  shells  "  of  the  Sauropsids. 
Among  the  most  complex  and  perfectly  adapted  membranes  or  shells, 
are  the  egg  cases  of  many  Elasmobranchs,  and  particularly  those  of 


GERM  CELLS  AND  THEIR  FORMATION 


109 


the  Holocephali  (Dean)  (Fig.  55),  which  often  remain  intact  and  func- 
tional, in  their  passive  way,  for  more  than  a  year.     In  some  cases  the 
egg  membranes  may  have  a  nutritive  value  and  may  augment  or  re- 
place the  food  supply  in  the  form  of  yolk  ;  the  eggs 
of   birds    and  snails  are  good  illustrations  of  this 
relation. 

We  must  now  trace  briefly  the  steps  leading 
to  the  formation  of  the  ova  and  spermatozoa 
as  the  highly  specialized  cells  we  have  de- 
scribed. In  the  lowest  Metazoa,  Sponges  and 
some  Hydroids,  the  germ  cells  are  scattered 
through  the  tissues  of  the  organism  as  sepa- 
rate, free  cells,  which  may  migrate  from  place 
to  place,  feeding  and  growing,  often  at  the 
expense  of  the  other  cells  (Fig.  56).  But  in 
other  Hydroids,  and  in  all  forms  above  these, 
the  germ  cells  are  localized  in  a  definite  re- 
productive tissue  and  organ,  or  series  of 
organs,  the  gonads  —  ovaries  and  testes.  The 
simplest  gonads  are  merely  masses  of  rapidly 
proliferating  cells  (Fig.  57),  usually  bordering 
a  cavity  which  is  the  coelom,  and  which  is 
supposed  to  be  primarily  this  reproductive 
cavity  simply.  In  most  of  the  higher  forms 
the  coelom  comes  to  have  many  secondary  re- 
lations, and  forms  in  addition  to  the  repro- 
ductive and  other  smaller  cavities,  the  very 
extensive  body  cavity.  In  the  embryos  of 
the  Craniates  there  is  a  pair  of  longitudinal 
ridges,  either  side  of  the  attachment  of  the  Fl?-  f?;""^ 

capsule  of  the  Holo 

dorsal  mesentery,  through  a  considerable  ex-  cephaian,    Chimera 


tent  of  the  body  cavity;  these  are  the  genital 

ridges,  and  the  peritoneum  covering  these  be-  natural  size.    After 

comes  thickened  by  the  enlargement  and  pro- 

liferation of  the  cells  (Fig.  58).     These  are  the  rudiments  of 

the  gonads.     The  cells  composing  these  rudiments  are  often  of 

two  kinds.     Some  of  them,  indifferent  cells  composing  in  gen- 


110 


GENERAL  EMBRYOLOGY 


FIG.  56. — Origin  of  the  germ  cells  in  the  Hydro-medusa,  Cladonema.  From 
Wilson,  "Cell,"  after  Weismann.  A.  Young  stage:  section  through  the  wall  of 
the  manubrium.  Ova  developing  in  the  ectoderm,  ec;  en,  endoderm.  B.  Older 
stage,  showing  ova,  o,  and  nutritive  cells,  n.  The  ova  contain  small  nuclei  prob- 
ably derived  from  ingested  nutritive  cells. 


FIG.  57. — Diagram  of  the  structure  of  the.  developing  ovary  of  the  Annulate, 
Amphitrite  rubra.  From  Korschelt  and  Heider,  after  E.  Meyer,  g.dr.,  rudiment 
of  ovary;  g.e.,  germinal  epithelium;  g.z.,  fully  formed  ova,  scattering;  pm.,  perit- 
oneum; V.v.,  vas  ventrale. 


GERM  CELLS  AND  THEIR  FORMATION 


111 


eral  the  frame-work  of  the  gonad,  have  been  formed  in  situ 
from  the  proliferating  peritoneal  cells.  Others,  the  primordial 
germ  cells,  are  often  first  distinguishable  in  some  other  region  of 
the  developing  embryo  (Fig.  59,  A);  they  then  make  their  way 
into  this  germinal  epithelium  as  development  proceeds. 

The  gonad  may  consist  almost  entirely  of  the  reproductive 
cells  proper,  and  may  be  then  a  more  or  less  periodic  structure, 


FIG.  58. — A.  Part  of  a  section  through  the  body  of  a  young  lizard,  Lacerta 
agilis,  showing  the  genital  ridges  and  associated  structures.  B.  Genital  ridge, 
enlarged,  showing  young  follicles  containing  ova.  From  Korschelt  and  Heider, 
after  Braun.  ao,  dorsal  aorta;  V,  cardinal  vein;  ms,  mesentery. 

almost  if  not  quite  disappearing  between  the  periods  of  repro- 
ductive activity.  Or  i.t  may  be  a  permanent  organ  of  con- 
siderable though  varying  size,  consisting  of  a  complex  stroma 
of  a  variety  of  cells — nutritive,  vascular,  nervous,  connective 
tissue,  and  other  cells,  in  addition  to  the  true  germ  cells.  When 
highly  developed  the  gonad  may  also  contain  various  cavities, 
of  ccelomic  character,  into  which  the  germ  cells  are  passed  when 
ripe.  Thence  they  may  pass  directly  into  the  body  cavity 
from  which  exit  is  made  to  the  outside  through  simple  perfora- 


112 


GENERAL  EMBRYOLOGY 


tions  in  the  body  wall  or  through  special  tubes  or  ducts.  In 
the  testes  these  ducts  may  lead  directly  to  the  outside  from  the 
cavities  of  the  gonads.  The  structure  of  the  gonad  can  nearly 
always  be  reduced  to  that  of  a  complexly  folded  epithelium 
the  essential  elements  in  which  are  the  germ  cells;  the  coelomic 
surface  is  the  free  surface  of  the  germinal  epithelium.  This 
relation  becomes  important  in  describing  the  fundamental 
morphology  of  the  germ  cell. 

It  is  a  question  whether  the  germ  cells  are  to  be  considered  as 
originally  undifferentiated  cells,  which  become  modified  during 
the  life  of  the  organism  for  the  reproductive  function,  or 
whether  they  are  set  apart  from  the  beginning  of  the  organism's 
multicellular  existence  as  reproductive  cells,  and  become  visibly 
modified  only  in  later  stages.  In  those  few  forms  where  the 


FIG.  59. — A.  Section  through  an  early  embryo  of  the  Teleost,  Micrometrus 
aggregatus,  showing  the  distinct  germ  cells.  After  Eigenmann.  ec,  ectoderm; 
en,  endoderm;  g,  germ  cells;" so,  somatic  layer  of  mesoderm;  sp,  splanchnic  layer 
of  mesoderm.  B.  Section  through  forty-cell  stage  of  the  Crustacean,  Cyclops 
brevicornis,  showing,  g,  the  cell  that  gives  rise  to  the  germ  cells,  en,  the  primi- 
tive endoderm  cell  in  the  process  of  its  first  division.  After  Hacker. 

germ  cells  are  diffused  it  seems  that  any  tissue  cell  which  is  not 
completely  specialized  in  some  other  direction  may  assume 
reproductive  characters.  In  forms  which  develop  special 
gonads,  however,  there  are  many  reasons  for  believing  that  the 
g£rm  cells  are  always  to  be  distinguished  as  such  very  early  in 
the  history  of  the  individual  organism.  In  A  scans  the  history 
of  the  primordial  germ  cells  'has  been  traced  back  to  one  of  the 
two  cells  resulting  from  the  very  first  division  of  the  fertilized 
ovum;  not  all  of  the  descendants  of  this  cell  are  germinal  how- 


GERM  CELLS  AND  THEIR  FORMATION  113 

ever  (Fig.  32).  In  some  of  the  bony  fishes  germinal  cells  are 
recognizable  in  the  fifth  cell  generation,  i.e..  in  the  thirty-two 
cell  stage.  And  in  many  other  forms,  including  some  of  the 
Mammalia,  the  germinal  cells  can  be  distinguished  from  the 
somatic  cells  very  early,  even  in  the  blastula  (Fig.  59).  This 
may  indicate  that,  although  not  visibly  distinct,  the  germ 
tissue  is,  after  all,  in  reality  distinct  from  the  somatic,  in  most 
if  not  in  all  forms.  It  would  seem  more  consistent  with  the 
present  conception  of  development,  however,  to  say  that  this 
distinction  exists  only  potentially  and  comes  about  as  a  real 
differentiation  in  the  developing  organism,  for  however  early 
this  differentiation  may  occur,  a  stage  is  always  found  where 
germinal  and  somatic  substances  are  contained  undifferentiated 
within  a  single  cell  and  are  then  indistinguishable. 

The  visible  distinction  between  the  gonads  of  different  sexes 
may  occur  very  early.  In  some  forms  this  distinction  between 
sexes  can  be  made  out  in  the  fertilized  ovum.  And  in  many 
forms  the  two  kinds  of  gonads  can  be  distinguished  soon  after 
they  are  first  marked  out,  though  there  is  reason  even  here  for 
supposing  that  the  distinction  is  really,  though  not  visibly, 
present  in  the  fertilized  egg. 

The  processes  involved  in  the  later  differentiation  or  histo- 
genesis  of  the  eggs  and  spermatozoa  are  collectively  termed 
oogenesis  and  spermatogenesis  respectively.  They  are  con- 
veniently divided  for  description  into  three  periods  or  phases. 
These  are  (1)  the  period  of  cell  multiplication,  during  which  the 
simple  epithelial  cells,  or  primordial  germ  cells,  divide  more  or 
less  continuously,  increasing  the  bulk  of  the  gonad;  (2)  the 
period  of  growth,  when  cell  division  is  less  rapid  or  altogether 
inhibited,  and  the  cells  enlarge  rapidly,  the  egg-forming  cells 
much  more  considerably  than  those  forming  the  spermatozoa; 
(3)  the  period  of  maturation,  when  the  germ  cell  nuclei  undergo 
profound  modifications  during  their  last  two  divisions  as  germ 
cells.  Sometimes  the  terms  oogenesis  and  spermatogenesis  are 
used  to  indicate  only  the  events  of  this  third  period,  which  are 
of  such  importance  that  we  shall  make  them  the  subject  of  the 


114 


GENERAL  EMBRYOLOGY 


next  entire  chapter.  In  the  history  of  the  spermatozoa  a  fourth 
period  is  to  be  distinguished,  namely,  the  period  of  transforma- 
tion or  metamorphosis;  for  the  highly  differentiated  structure  of 
the  spermatozoon  is  rapidly  assumed  after  the  process  of 
maturation  is  completed.  This  period  is  not  marked  in  the 
history  of  the  ovum,  for  this,  with  the  exception  of  its  unusual 
size,  is  not  so  markedly  differentiated  in  structure. 

During  the  first  of  these  periods,  that  of  multiplication,  the 
cells  of  the  reproductive  tissue  are  termed  odgonia  and  sper- 

Primordial  Germ 

Cell  ("Primitive 

Ovum"). 


Period  of  Multiplica- 
tion .  *  c  h  r  o  m  o- 
somes.  (The  num- 
ber of  cell  genera- 
tions is  much 
greater  than  indi- 
cated here.) 


Period  of  Growth 
s  chromosomes. 


Period  of  Matura- 
tion. ^  chromo- 
somes. 


Oogonia. 


FIG.   60. — Diagram   of  the   chief   events   of   06 genesis. 
Compare  with  Fig.  61. 


Primary  Oocyte. 


Secondary  Oocyte 
and  First  Polar  Body. 


Mature     Ovum    and 
Three  Polar  Bodies. 


Adapted   from   Boveri. 


matogonia,  and  of  these  there  may  be  a  great  many  generations 
during  this  period,  before  growth  commences.  As  the  oogonia 
and  spermatogonia  become  older,  division  becomes  slower  and 
ceases  as  the  cells  enter  upon  their  growth  period.  At  the  close 
of  the  growth  period,  while  still  contained  within  the  ovary  or 
testis,  the  cells  are  known  respectively  as  the  primary  odcytes, 
or  ovarian  eggs,  and  the  primary  spermatocytes.  From  this 
point  onward  the  histories  of  the  eggs  and  sperm  are  not  quite 
identical,  although  entirely  equivalent  (Platner,  0.  Hertwig). 


GERM  CELLS  AND  THEIR  FORMATION 


115 


As  said  above,  the  chief  events  concern  the  nuclear  structure 
and  we  can  only  point  out  here  that  during  the  period  of  matu- 
ration two  more  cell  divisions  occur. 

In  the  testis  each  primary  spermatocyte  divides  once,  form- 
ing two  cells  called  the  secondary  spermatocytes,  and  then  each 
of  these  divides  again,  forming  altogether  four  cells  called  the 


Primordial  Germ 
Cell. 


Period  of  Multiplica- 
tion, s  chromo- 
somes.  (The  number 
of  cell  generations 
much  gi  eater  than  is 

indicated  here.)  _  _  

Spermatogonia. 


Period  of  Growth. 
s  chromosomes. 


Period     of     Matura- 


tion.   —  chromo- 


Period  of   Metamor- 
phosis, ^chromo- 


Primary  Spermato- 
cyte. 


Secondary       Sper- 
matocytes. 


Spermatids. 


Spermatozoa. 


FIG.    61. — Diagram    of   the    chief   events    of   spermatogenesis.     Adapted   from 

Boveri. 

spermatids.  These  are  all  alike  and  each  becomes  metamor- 
phosed into  a  spermatozoon  without  further  division.  Thus 
are  formed,  from  each  primary  spermatocyte,  four  similar 
spermatozoa.  In  the  ovary,  or  sometimes  after  the  primary 
oocyte  has  left  the  ovary,  this  too  divides,  but  here  the  division 
is  very  unequal,  resulting  in  the  formation  of  one  large  cell, 
the  secondary  oocyte,  and  one  very  small  cell,  called  the  first 
polar  body.  Typically  each  of  these  then  divides  again.  The 
secondary  oocyte  divides  unequally  as  before,  forming  a  large 


116 


GENERAL  EMBRYOLOGY 


cell  the  mature  ovum,  and  another  small  cell,  the  second  polar 
body.  Meanwhile  the  first  polar  body  divides  equally  forming 
two  similar  polar  bodies.  In  some  cases  the  division  of  the 
first  .polar  body  is  suppressed.  Thus  each  primary  odcyte 
typically  gives  rise,  like  the  primary  spermatocyte,  to  four  cells, 
but  these  are  not  all  alike  in  form  and  size,  although  they  are 
fundamentally  equivalent,  i.e.,  homologous,  to  each  other,  and 
to  the  four  spermatids.  Of  these  four  cells,  however,  only  one, 
the  ovum,  is  functional;  the  polar  bodies  degenerate  without 
functioning.  The  parallel  events  of  spermatogenesis  and 
oogenesis  are  shown  diagrammatically  in  Figs.  60-61.  (See 
also  Figs.  76,  90,  94.) 


o'v 


FIG.  62.— Longitudinal  section  through  the  ovary  of  the  Copepod,  Cantho- 
camptus.  From  Wilson,  "Cell,"  after  Hacker,  og,  the  youngest  germ-cells  or 
oogonia  (dividing  at  og.2)  ;  a,  upper  part  of  the  growth-zone;  oc,  oocyte^or  growing 
ovarian  egg;  ov,  fully  formed  egg,  with  double  chromatin-rods. 

All  these  processes  may  be  going  on  in  the  ovary  or  testis  at 
the  same  time,  occurring  progressively  from  the  basement 
membrane  of  the  germinal  epithelium  toward  its  free  surface, 
so  that  a  section  through  such  an  epithelium  shows  practically 
every  step  in  the  history  of  a  single  cell  (Figs.  62,  68,  69). 
Before  considering  in  detail  the  nuclear  changes  involved  in 
the  maturation  processes  we  must  consider  the  more  important 
facts  concerning  the  history  of  the  cytoplasmic  parts  during 
this  phase  of  the  genesis  of  the  germ  cells. 


GERM  CELLS  AND  THEIR  FORMATION 


117 


FIG.  63. — Diagrams  of  the  egg-tubes  (ovaries)  of  Insects.  After  Korschelt 
and  Heider.  A.  Orthoptera,  without  groups  of  nutritive  cells.  B,  Coleoptera, 
with  many  such  groups.  C.  Hemiptera,  with  terminal  nutritive  group  and 
nutritive  channels  extending  to  the  ova.  c,  nutritive  channel;  /,  egg  follicle; 
m,  zone  of  multiplication;  n,  nutritive  cells;  o,  ova. 


118 


GENERAL  EMBRYOLOGY 


In  the  growth  of  the  egg  the  chief  aspects  are  those  associated 
with  nutritive  relations  of  the  developing  ovum  to  the  adjacent 
cells,  especially  in  those  forms  whose  eggs  contain  a  considerable 
amount  of  yolk.  In  the  non-localized  ovary  such  as  that  of  the 
Sponges  and  some  Hydroids,  the  ovum  grows  at  the  expense 
of  whatever  cells  happen  to  be  adjacent  to  it  (Fig.  56).  In  the 
common  Hydra,  as  the  ovum  grows  to  be 
considerably  larger  than  these  tissue  cells, 
it  becomes  amoeboid  and  actually  ingests 
these  neighboring  cells,  digesting  their  sub- 
stance and  growing  rapidly  (Fig.  44).  The 
nuclei  of  these  ingested  cells  are  relatively 
indigestible  and  remain  for  some  time  scat- 
tered through  the  egg  cytoplasm.  Among 
all  those  forms  with  definitely  localized 
ovaries  growth  of  the  ova  is  accomplished 
very  differently.  When  the  eggs  are  small 
and  contain  relatively  little  food,  no 
special  nutritive  mechanism  is  developed, 
the  egg  forming  the  food  substances  in  its 
own  cytoplasm  from  materials  drawn  from 
the  circulating  fluids  in  the  cavities  of 
the  ovary.  Such  eggs  develop  independ- 
ently of  the  neighboring  cells  intermingled 
with  the  ova. 

the  fu»    formed  e 


part  of  the  egg-tube  amounts  of  food  substance  this  is  usually 

(ovary)  of    the    beetle,      .        .        ,    .  -  .  .    f 

Dytiscus      marginaiis.  obtained  by  one  oi  two  chief  methods.     In 
After     Korscheit.     n,  fae  simplest  cases  certain  of  the  ovarian 

groups      of      nutritive  *  t 

cells;  o,  ovum  contain-  cells  adjoining  the  egg  take  on  the  charac- 

ing     amoeboid    nucleus    ,      •   »'•  /«  77          rm  -,i 

partly  surrounded  by  tenstics  of  nur  SB  cells.    These  may  either 
nutritive  substance  contribute  their  own  substance  directly  to 

(deutoplasm). 

the  ovum  or  they  may  become  intensely 
active,  forming  deutoplasm  which  is  then  drawn  from  them  by 
the  growing  ovum  (Figs.  63,  64).  There  may  be  a  single 
nurse  cell  for  each  ovum  (Fig.  65),  or  the  nurse  cells  may 
be  scattered  irregularly  through  the  ovary  so  that  several 


GERM  CELLS  AND  THEIR  FORMATION 


119 


may  be  related  to  each  ovum.  The  nurse  cells  in  many  or 
even  in  most  cases,  are  cells  which  were  potentially  germ 
cells,  but  which  have  lost  their  germinal  potentiality  and 


FIG.  65. — Sections  through  ovarian  ova  and  nurse  cells  in  the  Annulate, 
Ophryotrocha.  From  Wilson,  "Cell,"  after  Korschelt.  A.  Young  stage,  the 
nurse  cell,  n,  larger  than  the  egg.  B.  Growth  of  the  ovum,  o.  C.  Late  stage, 
the  nurse  cell  degenerating. 

become  wholly  nutritive,  contributing  to  the  formation  of 
the  true  germ  cell,  degenerating  and  disappearing  completely 
after  the  ovum  is  grown  and  has  left  the  ovary.  In  the  other 


A  B 

FIG.  66. — Sections  through  ovarian  eggs  and  their  follicles  in,  A,  young  magpie; 
B,  newly  born  child.     From  Wilson,   "Cell,"  after   Mertens. 

cases,  which  are  commonest  among  the  Vertebrates,  almost  uni- 
versal among  them  in  fact,  the  ovarian  cells  adjoining  the  ovum 
• — themselves  potentially  germ  cells  originally,  form  around  the 
ovum  a  definite  layer  termed  the  follicle  (Fig.  66).  The  follicle 


120 


GENERAL  EMBRYOLOGY 


cells  have  the  arrangement  of  an  epithelium;  they  may  form 
either  a  single  layered,  simple  epithelium  or  in  other  cases  a 
many  layered  stratified  epithelium  (Fig.  43).  They  not  only 
provide  for  the  nutrition  of  the  enlarging  ovum,  with  which 
they  are  frequently  connected  by  definite  intercellular  proto- 
plasmic strands,  but  toward  the  close  of  the  growth  period  of 

the  egg  they  may  become  secre- 
tory and  form  certain  egg  envel- 
opes of  the  secondary  type,  i.e., 
chorionic.  We  have  already  men- 
tioned how  the  micropyle  is 
formed  through  the  chorion  and 
vitelline  membrane  by  the  inser- 
tion of  one  of  these  follicle  cells 
with  a  long  process,  preventing  the 
membranes  from  forming  at  that 
point.  When  the  eggs  are  fully 
formed  and.  ready  to  be  laid  the 
follicle  ruptures  allowing  the  eggs 
to  escape  freely.  Often  there  de- 

r.  *«•••• 

VelopS     in     the    f  ollicle    a    definite 

region  along  which  it  bursts;  this 
weakened  region  is  called  the 

.          .        . 
ClCCLinX    (e.g.,    Common  lOWl).       In 

f  instances  some  of  the  follicle 
cells  are  actually  taken  into  the 
egg  and  absorbed,  much  as  in  the  case  of  those  ova  which  in- 
gest adjacent  interstitial  cells.  This  is  the  case  in  many  of 
the  Tunicata  where  some  of  the  so-called  "test-cells"  lose 
their  cell  outlines  and  are  directly  taken  into  the  cytoplasm 
of  the  egg;  then*  nuclei  remain  distinct  for  some  time  (Fig.  67). 
The  remaining  test  cells  then  form  a  distinct  follicle  outside 
the  whole  structure.  These  nuclei  no  longer  function  as 
nuclei  of  course;  as  the  growth  of  the  egg  is  completed  they 
are  extruded  again,  along  with  a  portion  of  the  superficial  pro- 
toplasm forming  then  a  thick  vitelline  membrane  resem- 
bling a  chorion  and  often  so  called. 


FIG.  67.-Section  through  the 
egg    of    the   Tunicate,   Cynthia 

partita.     After    Conklm.      In    the 
periphery  of  the  egg  are  the  nuclei 

r  %£ 
later  history  see  Fig.  91).    x  357. 

e,  exoplasm   or   cortical  layer;    n, 
egg    nucleus   or   germinal  vesicle, 

with  large  nucieoius;  t,  nuclei  of 

test  cells  or  follicle  cells;  y,  yolk. 


GERM  CELLS  AND  THEIR  FORMATION          121 

Turning  now  to  the  formation  of  the  sperm  we  find,  as  we 
should  expect,  processes  on  the  whole  entirely  comparable 
with  those  of  egg  formation.  The  chief  differences  result  from 
the  fact  that  in  the  ovary  conditions  are  associated  with  the 
formation  of  a  few  relatively  large  cells,  while  in  the  testis 
small  cells  are  formed,  but  in  very  large  numbers.  In  Sponges 
and  Hydroids  there  is  the  same  non-localized  formation  of  the 
sperm  as  of  the  ova,  the  germ  cells  being  distinguished  not  so 
much  by  position  as  by  size.  Apparently  any  ectoderm  cell 
may  enlarge  and  become  reproductive.  In  all  forms  above 
these  there  are  definite  testes.  Among  many  Coelenterates 
and  Echinoderms  the  testis  is  composed  purely  of  germ  cells,  but 
usually  the  testis,  like  the  ovary,  contains  other  interstitial  or 
accessory  cells,  and  frequently  these  are  directly  nutritive  in 
function.  The  general  structure  of  the  testis  differs  from  that 
of  the  ovary  in  that  its  epithelium  is  thrown  into  folds,  forming 
either  simple  columns  or  acini,  each  with  an  efferent  duct  or 
pathway  which  is  to  be  considered  coelomic  in  origin.  In  the 
testicular  epithelium,  which  is  ordinarily  reducible  to  the  strati- 
fied type,  we  find  sperm  cells  in  all  stages  of  formation,  from 
spermatogonia  to  fully  formed  spermatozoa.  As  we  have 
already  said,  the  process  of  sperm  formation  is  usually  continu- 
ous, though  frequently  periodic  (seasonal)  in  its  rate  or  inten- 
sity. When  the  testis  is  composed  of  acini,  each  is  usually 
surrounded  by  a  follicle,  equivalent  in  function  to  the  egg  folli- 
cle. But  when  arranged  in  lobules  and  columns,  each  of  which 
may  be  derived  from  a  single  primordial  cell  or  prespermatogon- 
iunij  the  nutritive  cells  do  not  show  this  follicular  arrangement, 
but  are  fewer  in  number  and  scattered  along  the  basement 
membrane  of  the  epithelium,  or  along  the  connective  tissue 
partitions  bounding  the  spermatic  columns  of  the  lobule,  and  to 
some  extent  among  the  germ  cells  proper. 

Among  the  Craniates  the  typical  arrangement  is  that  shown 
in  Figs.  68,  69.  Here,  along  the  basement  membrane,  are  several 
generations  of  spermatogonia  with  the  scattered  nutritive 
basal  cells,  sometimes  called  also  the  Sertoli  cells,  usually  larger 
than  the  spermatogonia.  As  the  spermatogonia  increase  in 


122 


GENERAL  EMBRYOLOGY 


number,  through  continued  mitosis,  they  begin  to  increase  in 
size,  though  not  nearly  to  the  same  extent  that  the  oogonia  do. 
There  is  not  always  the  same  distinctness  between  the  phases 
of  multiplication  and  growth  here,  and  the  two  final  divisions 
of  the  full  grown  spermatogonial  cell,  then  known  as  the 


FIG.  68. — Diagram  of  a  section  through  part  of  the  testis  of  the  rat,  showing 
some  stages  in  spermatogenesis.  From  Korschelt  and  Heider,  after  Lenhossek. 
bz,  basal  cells;  spc,  spermatocytes;  spg,  spermatogonia;  spt,  spermatids;  spz, 
spermatozoa. 

primary  spermatocyte,  are  the  reducing  or  maturation  divisions. 
These  lead,  as  we  have  seen,  to  the  formation,  from  each  primary 
spermatocyte,  first  of  two  secondary  spermatocytes,  both  alike, 
and  then  to  four  spermatids,  all  alike.  The  cells  of  the  column 
then  become  arranged  so  that  groups  of  spermatids  become 
related  with  each  of  the  basal  cells,  which  often  leave  their 


GERM  CELLS  AND  THEIR  FORMATION 


123 


original  position  and  move  out 
toward  the  free  surface  of  the 
epithelium.  Through  their  at- 
tachment to  the  basal  cells  the 
spermatids  draw  a  supply  of  8 
food  and  energy  during  their 
rapid  and  extensive  metamor- 
phosis into  spermatozoa.  In 
some  cases  a  very  close  relation 
is  established  by  the  actual 
embedding  of  one  end  of  the 
spermatid  in  the  substance  of 
the  basal  cell.  It  should  be 
noticed  that  the  function  of  the 
follicle  or  basal  cells  of  the 
testis  is  to  supply  nutrition  to 
the  germ  cells,  not  so  much 
during  their  period  of  growth  as 
after  that  is  completed,  during 
the  period  of  metamorphosis; 
while  in  the  ovary  the  corre- 
sponding cells  function  during 
the  growth  period;  this  is  cor- 
related with  the  smaller  size 
of  the  spermatocyte  and  with 


FIG.  69. — Diagrammatic  outline  of 
the  spermatogenesis  of  the  rat  in  thirty- 
two  stages.  After  v.  Ebner.  Base- 
ment membrane  toward  the  left.  1-8. 
Period  of  multiplication  (the  number 
of  cell  generations  is  actually  very  large). 
9-18.  Period  of  growth.  19-24.  Period 
of  maturation.  25-32.  Period  of  meta- 
morphosis. 6,  basal  cells  or  Sertoli 
cells.  1-16.  Spermatogonia.  17,  18. 
Primary  spermatocytes  preparing  for 
division.  19.  First  spermatocyte  divi- 
sion. 20.  Secondary  spermatocytes. 
21.  Secondary  spermatocyte  division. 
22-25.  Spermatids.  26-31.  Trans- 
formation of  spermatids.  32.  Fully 
formed  spermatozoa. 


32 


124 


GENERAL  EMBRYOLOGY 


the  need  for  an  easily  available  food  supply  for  the   large 
number  of  sperm  cells  during  their  metamorphosis. 

Probably  the  most  interesting  phase  of  the  cytoplasmic  as- 
pects of  spermatogenesis  is  this  metamorphosis  of  the  sper- 


FIG.  70. — Formation  of  the  spermatozoon  in  Urodeles.  From  Wilson, 
"Cell,"  A—E,  Salamandra,  after  Meves;  F—K,  Amphiuma,  after  McGregor. 
A.  Spermatid  with  peripheral  pair  of  centrosomes  (c)  lying  outside  the  sphere, 
and  axial  filament.  B.  Centrosomes  near  the  nucleus,  outer  one  ring-shaped; 
a,  acrosome.  C.  Inner  centrosome  inside  the  nucleus,  enlarging  to  form  middle 
piece;  n,  nucleus.  D.  Portion  of  much  older  spermatid,  showing  divergence  of 
two  halves  of  the  ring,  r.  E.  Portion  of  mature  spermatozoon,  showing  upper 
half  of  ring  at  r,  and  the  axial  filament  proceeding  from  it.  F.  Spermatid  of 
Amphiuma,  showing  sphere-bridges  and  ring-shaped  mid-bodies.  G.  Later 
stage;  outer  centrosome  ring-shaped,  inner  one  double;  sphere,  s,  converted  into 
the  acrosome,  a.  H.  Migration  of  the  centrosomes.  /.  Middle-piece  at  base 
of  nucleus.  J.  Inner  centrosome  forms  the  end-knob  within  the  middle-piece, 
which  is  now  inside  the  nucleus.  K.  Enlargement  of  middle-piece,  m,  end-knob 
within  it;  elongation  of  the  ring. 

matids  into  spermatozoa.  After  growth  and  maturation  the 
spermatids  have  much  the  same  external  appearance  as  any 
typical  cell;  they  are  more  or  less  spherical  cells  with  a  pair  of 


GERM  CELLS  AND  THEIR  FORMATION          125 

centrosomes  or  centrioles,  and  a  large  spherical  nucleus  with 
a  dense  chromatin  network.  Internally  we  know  that  the 
nuclei  are  unlike  those  of  the  somatic  cells  on  account  of  the 

rt 

presence  of  only  -  chromosomes.     Without  any  further  divi- 
2 

sion  each  is  converted  into  the  special  form  of  the  spermatozoon 
typical  of  the  species.  While  the  details  of  this  metamorphosis 
vary  considerably  in  different  groups,  the  essentials  of  the 
process  are  everywhere  the  same.  The  spermatid  (Figs.  70,  A,  F; 
71,  A)  contains,  in  addition  to  the  typical  cell  organs  just  men- 
tioned, a  modified  region  of  the  cytoplasm  which  is  sometimes 
a  centrosphere  or  idiosome,  sometimes  of  rather  doubtful  charac- 
ter and  origin,  which  for  convenience  may  be  termed  the  sper- 
matosphere.  Close  by  lie  the  remains  of  the  last  division  spindle. 
The  spermatids  are  further  characterized  by  the  presence  of  a 
collection  of  chromidial  structures  termed  the  mitochondria 
(Fig.  71). 

The  details  of  the  metamorphosis  of  these  structures  into  the 
parts  of  the  spermatozoon  are  subject  to  wide  variation;  the 
following  account  is  based  upon  the  history  of  the  mammalian 
spermatid  (Fig.  71).  The  first  step  in  the  process  is  the  migra- 
tion of  the  centrosomes  to  the  surface  of  the  cell,  and  at  the 
same  time  the  migration  of  the  nucleus  to  the  opposite  side  of 
the  cell.  In  most  cases  it  is  difficult  to  determine  the  relation 
of  the  axis  thus  marked  out,  but  in  many  instances  this  is  per- 
pendicular to  the  basement  membrane  of  the  germinal  epithe- 
lium, thus  expressing  a  polarity  which  coincides  with  that  of 
the  ovum;  the  nucleus  lies  toward  the  membrane,  i.e.f  the 
attached  surface  of  the  cell,  the  centrosomes  toward  the  free 
surface.  As  the  centrosomes  and  nucleus  are  taking  these  new 
positions  the  spermatosphere  moves  up  to  the  nucleus  and 
around  it  to  the  side  opposite  the  centrosomes,  quite  to  the 
surface  of  the  spermatid.  The  two  centrosomes  now  separate, 
one  approaching  the  nucleus,  the  other  remaining  peripherally. 

Following  these  changes  in  the  relative  positions  of  the  parts 
come  the  real  modifications  of  structure.  The  nucleus  becomes 
elongated  or  ovoid,  and  the  chromatin  condenses,  first  into  a 


126 


GENERAL  EMBRYOLOGY 


FIG.  71. — Metamorphosis  of  the  spermatic!  of  the  guinea  pig,  Cavia  cobaya. 
After  Meves.  L.  Side  view,  showing  flattening  of  head,  a,  axial  filament; 
c,  centrosomes  (centrioles) ;  e,  neck,  containing  end  knobs  (proximal  centriole) ; 
k,  chromidial  "nebenkorper;"  m,  middle  piece;  n,  nucleus;  s,  centrosphere  (aero- 
some)  ;  u,  annulus  (posterior  portion  distal  centrosome) ;  v,  mitochondria  (partly 
becoming  a  portion  of  the  tail  envelope) ;  y,  cytoplasmic  portion  of  spermatid, 
being  thrown  off  in  K  and  L;  in  J,  K,  L  containing  mitochondrial  remains. 


GERM  CELLS  AND  THEIR  FORMATION  127 

heavy  reticulum,  and  then  into  a  dense  mass  in  which  no  visible 
structure  remains;  finally  it  acquires  the  form  of  the  head  of 
the  mature  spermatozoon.  The  spermatosphere  meanwhile  is 
largely  converted  into  the  acrosome  or  perforatorium,  at  the 
tip  of  the  elongated  nucleus;  a  smaller  portion  is  transformed 
into  a  very  delicate  envelope  covering  a  part  of  the  head,  in 
many  cases  scarcely  distinguishable  on  account  of  its  thinness. 
At  the  other  end  of  the  cell  a  fine  flagellum  begins  to  grow  out 
in  connection  with  the  peripheral  centrosome,  either  from  the 
substance  of  the  centrosome  itself,  or  from  the  cytoplasm  under 
its  influence.  Then  the  two  centrosomes  move  farther  in 
toward  the  nucleus.  In  the  simpler  cases  the  distal  centro- 
some now  divides  into  anterior  (toward  the  head)  and  posterior 
portions.  The  posterior  part  grows  out  peripherally  into  a 
rapidly  elongating  fiber  which  becomes  the  axial  filament  of  the 
flagellum,  while  at  its  base  it  becomes  ring-like;  then  through 
this  ring  or  annulus  the  axial  filament  grows  in,  finally  connect- 
ing with  the  anterior  portion  of  the  centrosome.  The  anterior 
portion  itself  remains  in  the  middle  piece  as  a  posterior  centro- 
some of  the  spermatozoon.  The  proximal  or  anterior  centro- 
some partly  disappears,  and  partly  is  converted  into  that  part 
of  the  middle  piece  which  connects  with  the  head  (the  neck). 

The  cytoplasmic  part  of  the  spermatid  seems  to  be  largely 
consumed  in  providing  the  energy  for  this  transformation,  in 
addition  to  that  drawn  from  the  nurse  or  Sertoli  cells.  But  the 
cytoplasmic  membranes  of  the  middle  piece  and  tail,  including 
the  undulatory  membrane  when  this  is  present,  are  derived 
directly  from  the  cytoplasm  of  the  spermatid.  The  mitochondria 
of  the  spermatid  seem  to  be  transformed  into  the  spiral  layer 
of  the  middle  piece.  In  many  instances,  particularly  among 
the  Mammals,  the  larger  part  of  the  cytoplasm  remains  for  a 
time  connected  with  the  middle  piece  of  the  developing  sper- 
matozoon and  then  is  cast  off,  and  finally  degenerates  without 
taking  any  further  part  in  the  structural  formation  of  the 
functional  sperm  cell.  The  chief  structural  correspondences 
between  spermatid  and  spermatozoon  are  shown  in  the  accom- 
panying table. 


128 


GENERAL  EMBRYOLOGY 


COMPARISON   OF   THE   STRUCTURES   OF   THE   SPERMATID   AND 
SPERMATOZOON 

With  particular  reference  to  the  mammalian  condition.     (Partly  from 

Gegenbaur-Fiirbringer,  Lehrbuch  der  Anatomie  des  Menschen, 

Leipzig,  1909) 


SPERMATID 
Nucleus. 

Spermatosphere  (centre- 
sphere)  . 

Proximal  centrosome   (cen- 
triole) . 


Distal  centrosome  (cen- 
triole) . 

(a)  Anterior  portion. 

(b)  Posterior  portion. 


Cell  body. 


SPERMATOZOON 


Head. 


Mitochondria. 


Acrosome  (perforat-orium)  and  sheath 
covering  the  anterior  part  of  the  head. 

Forms  an  undifferentiated  part  of  the 
middle  piece;  in  Mammals,  the  neck. 
In  part  may  disappear. 


Centrosome  of  middle  piece. 
Annulus  and   axial   filament  of  middle 
piece  and  tail. 

Partly  used  as  source  of  energy  during 
metamorphosis.  Partly  thrown  off. 
Remainder  forms  cytoplasmic  envel- 
opes of  middle  piece  and  tail  (includ- 
ing undulatory  membrane) . 

Spiral  membrane  of  middle  piece. 


Whatever  the  details  of  the  metamorphosis  of  the  spermatid 
may  be,  the  facts  of  essential  importance  are  always  identical. 
These  are,  that  the  nucleus  of  the  spermatid  is  directly  trans- 
formed into  the  head  of  the  spermatozoon;  the  centrosomes 
of  the  spermatid  become  the  centrosomes  and  kinoplasmic 
structures  of  the  spermatozoon  and  are  contained  within  the 
middle  piece,  or  partly  in  the  tail;  the  cytoplasm  of  the  sper- 
matid in  part  goes  to  form  a  thin  cytoplasmic  investment  of  the 
spermatozoon  or  is  in  part  cast  off. 

The  fully  formed  spermatozoa  now  lose  connection  with  the 


GERM  CELLS  AND  THEIR  FORMATION          129 

nurse  cells  and  pass  by  way  of  the  canals  or  ducts  of  the  testis 
into  the  efferent  reproductive  ducts,  vasa  efferentia  and  vasa 
deferentia,  to  the  outside,  either  directly,  or  after  being  stored 
for  a  time  in  special  cavities  such  as  the  seminal  vesicles.  The 
sperm  may  remain  alive  in  these  storage  cavities  for  a  long 
time,  awaiting  the  period  of  extrusion,  upon  the  approach  of 
which  they  become  very  active. 

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CHAPTER  IV 
MATURATION 

Ix  this  chapter  we  shall  describe  certain  events  which  are  in 
reality  essential  steps  in  the  processes  of  oogenesis  and  sper- 
matogenesis,  namely,  the  maturing  of  the  nuclei  of  the  definitive 
germ  cells.  In  animals  these  maturation  processes  are  the 
final  steps  in  the  complete  specialization  of  the  germ  cells,  and 
must  be  accomplished  before  the  two  gametes  can  fuse  com- 
pletely and  thus  begin  the  life  of  the  "new"  organism  as  an 
individual.  As  a  matter  of  fact,  the  processes  of  maturation 
may  be  inaugurated  before  the  growth  and  differentiation  of 
the  germ  cells  are  entirely  completed,  and  these  processes  may 
then  all  go  on  together.  They  are  considered  here  separately, 
and  without  complete  regard  for  then*  normal  time  relations, 
partly  as  a  matter  of  convenience,  looking  toward  simplicity 
of  description,  and  partly  to  emphasize  their  great  importance 
as  a  period  in  the  development  of  the  organism.  Such  a  sepa- 
ration is  easy  because  the  maturation  processes  are  not  visibly 
connected  with  the  genesis  of  the  germ  cells  as  such,  for,  mor- 
phologically at  any  rate,  they  concern  only  or  chiefly  the 
nuclei  alone;  the  accompanying  cytoplasmic  modifications  of 
structure  have  already  been  described. 

That  morphological  characteristic  chiefly  distinguishing  the 
fully  matured  germ  cells  is  the  possession  of  but  one-half  the 
number  of  chromosomes,  and  of  but  a  smaller  fraction  of  the 
amount  of  chromatic  material,  possessed  by  the  somatic  cells 
(Van  Beneden) .  We  should  include  under  the  term  maturation, 
the  whole  series  of  events  leading  to  this  reduction  in  number 
of  chromosomes  and  amount  of  chromatin.  It  should  be  noted, 
however,  that  the  terms  "  oogenesis"  and  " spermatogenesis " 
have  sometimes  been  used  in  a  restricted  sense  to  mean  what 
we  here  term  "  maturation,"  but  we  have  understood  the  former 

131 


132  GENERAL  EMBRYOLOGY 

terms  to  include  the  whole  history  of  the  germ  cells  up  to 
the  time  of  their  formation  as  completely  specialized  struc- 
tures, and  maturation  therefore  becomes  a  phase  in  06-  and 
spermatogenesis. 

It  is  a  familiar  fact  that  in  fertilization  the  union  of  an  egg 
nucleus  and  a  sperm  nucleus  is  an  essential  step.  The  repeated 
union  of  nuclei  of  the  usual  type,  in  this  way,  would  result  in 
rapid  and  limitless  increase  in  chromatic  elements  and  material, 
but  for  the  operation  of  some  mechanism  preventing  such  an 
accumulation,  and  yet  permitting  the  fusion  of  germ-cell  nuclei. 
Maturation  is  such  a  process;  but  it  is  much  more  than  this. 
Consideration  of  all  the  phenomena  of  maturation  raises  many 
questions,  important,  even  fundamental,  in  their  biological 
significance.  The  full  meaning  of  the  phenomena  can  be 
appreciated  only  in  connection  with  the  fertilization  processes 
to  which  they  are  introductory;  we  may  most  profitably,  there- 
fore, postpone  much  of  our  discussion  of  the  general  significance 
of  maturation  until  we  have  become  acquainted  with  the 
process  of  fertilization. 

At  this  time,  then,  we  shall  describe  the  essential  facts  of 
maturation  as  we  find  them  in  typical,  in  some  respects  perhaps 
schematized,  form,  together  with  a  brief  comparative  account 
of  some  maturation  processes  in  a  few  special  instances.  Then 
after  a  similar  account  of  fertilization  in  the  next  chapter,  we 
shall  be  in  position  to  consider  some  of  the  general  aspects  of 
both  these  processes  taken  together.  The  events  of  maturation 
and  fertilization  are  really  closely  related  in  time,  as  well  as  in 
significance.  While  the  spermatozoa  are  always  fully  mature 
before  they  enter  the  egg  cells,  the  entrance  of  the  sperm  may 
occur  either  before,  during,  or  after  the  maturation  of  the  ovum, 
although  of  course  the  essential  step  in  fertilization,  namely,  the 
union  of  the  nuclei,  does  not  occur  (excepting  in  some  Protozoa 
and  plants)  until  after  the  maturation  of  the  egg  nucleus  is 
completed. 

The  maturation  of  the  germ  cells  is  accomplished,  in  the  Meta- 
zoa,  by  a  modification  of  a  mechanism  common  to  all  cells  and 
already  familiar,  namely,  mitosis.  But  the  cell  divisions  which 


MATURATION  133 

occur  here  are  of  a  very  special  form,  not  found  in  the  history 
of  other  kinds  of  cells.  These  unusual  mitoses  are  known  as 
the  meiotic  (maiotic,  Farmer  and  Moore),  or  reducing  divisions, 
and  their  chief  peculiarity  consists  in  the  fact  that  they  result 
in  the  formation  of  daughter  nuclei  containing  the  reduced 
or  haploid  number  of  chromosomes. 

We  shall  review  first  the  maturation  of  the  spermatozoon,  as 
this  is  less  modified  than  the  ovum,  from  those  conditions  which 
we  regard  as  typical.  Throughout  the  multiplication  divisions 
of  the  spermatogonia,  the  mitoses  are  all  of  the  usual  character, 
except  that  the  mitotic  figures  are  relatively  larger  than  in  the 
somatic  divisions.  The  number  of  the  chromosomes  is  the 
same  as  in  the  somatic  cells  of  the  same  organism;  this  is  spoken 
of  as  the  diploid  number.  In  many,  perhaps  most,  organisms 
the  chromosomes  differ  from  those  of  the  somatic  cells  in  form 
and  size  characters,  so  that  the  germinal  tissue  can  usually  be 
identified;  the  germinal  tissue  nuclei  are  in  general  larger  and 
richer  in  chromatin  than  those  of  somatic  tissues,  a  difference 
which,  as  previously  noted,  in  a  few  forms  (Ascaris,  Cyclops, 
some  Teleosts)  can  be  traced  from  very  early  cleavage  stages. 
But  the  constitution  of  the  nuclei  which  pass  into  the  inter- 
kinesis  after  the  last  spermatogonial  division,  i.e.,  into  the 
primary  spermatocyte  nucleus,  is  essentially  normal.  Some- 
times certain  peculiarities  become  noticeable  during  the  growth 
period  of  these  cells;  the  nucleus  frequently  does  not  remain  in 
the  typical  "resting"  condition,  but  forms  a  more  or  less  dis- 
tinct spireme  (leptonema,  Winiwarter),  and  sometimes,  even  at 
the  beginning  of  this  stage,  there  may  be  a  fission  of  the  chro- 
matin granules,  forming  a  sort  of  double  spireme  (Fig.  73). 
At  the  close  of  this  growth  period,  when  the  primary  spermato- 
cyte prepares  to  divide,  the  nucleus  begins  to  show  a  very 
unusual  condition.  The  nucleus  itself  remains  large,  but  the 
chromatin,  as  it  begins  to  form  a  spireme,  in  those  cases  wThere 
this  has  not  formed  previously,  condenses  at  one  side  of  the 
nucleus,  in  the  vicinity  of  the  nucleolus,  usually  in  the  region 
near  the  centrosomes  and  perhaps  through  their  influence 
(Schonfeld),  into  a  dense  mass  in  which  little  structure  can  be 


134 


GENERAL  EMBRYOLOGY 


G 


FIG.  72. — Early  stages  in  the  maturation  of  the  Dipnoan,  Lepidosiren  paradoxa. 
After  Agar.  X  933.  A.  Polar  view 'of  equatorial  plate  of  late  spermatogonium, 
showing  size  and  form  differentiation  and  pairing  of  chromosomes.  B.  Spireme 
stage  (leptonema)  of  primary  spermatocyte.  Only  a  few  of  the  threads  are 
shown.  C.  Nearly  polar  view,  showing  beginning  of  longitudinal  fusion  of 
chromatin  threads;  beginning  of  synapsis  (zygonema).  D.  Nearly  polar  view 
of  "bouquet  stage"  (pachynema).  The  threads  are  fused  and  condensed. 
E.  Polar  view  showing  beginning  of  contraction  (synizesis)  and  splitting  of  the 
chromosomes  (diplonema).  Most  of  the  thickened  threads  have  split  apart 
except  terminally,  where  they  remain  fused,  forming  rings.  In  some,  the 


MATURATION  135 

made  out  (Figs.  72,  73).  This  stage  is  called  the  contraction 
phase,  or  synizesis  (McClung)  (pachynema,  Winiwarter) .  In 
some  cases  synizesis  may  occur  near  the  beginning  of  the  growth 
period,  throughout  which  the  nucleus  then  remains  in  this 
condition. 

When  growth  of  the  spermatocyte  is  completed  this  knot  of 
chromatin  begins  to  disentangle  and  the  spireme  again  becomes 
visible.  This  later  spireme  is  not  continuous,  however,  but  is 
of  the  segmented  type  (Fig.  72),  and  the  number  of  segments, 
i.e.,  of  chromosomes,  is  but  one-half  the  number  of  chromosomes 
that  went  into  the  nucleus  at  the  close  of  the  preceding  division.  As 
regards  the  chromatic  structures,  this  is  the  essential  point  in 
the  whole  maturation  process;  the  number  of  chromosomes 
formed  in  the  prophase  of  the  first  spermatocyte  division  is 
reduced  to  one-half  the  somatic  number.  This  numerical 
reduction  of  the  chromosomes  is  brought  about  in  most,  if  not 
in  all  cases  so  far  known,  by  an  actual  fusion,  by  twos,  of  the 
chromosomes  contained  in  the  last  spermatogonial  nucleus. 
This  fusion  of  pairs  of  chromosomes  is  termed  synapsis  (Moore, 
McClung)  or  syndesis  (Hacker)  (zygonema,  Winiwarter),  and 
the  resulting  units  are  thus  double  or  bivaknt  chromosomes.  It 
seems  entirely  likely,  if  not  definitely  established,  that  the  pah's 
of  chromosomes  which  come  together  in  synaptic  fusion  are 
each  composed  of  one  chromosome  derived  from  the  male 
parent,  and  the  corresponding  chromosome  derived  from  the 
female  parent,  similar  in  size,  and  form,  and  also  in  function  if 
we  assume  the  fact  of  chromosomal  specificity  (Montgomery). 
These  two  groups  of  similar  elements  came  into  a  single  nucleus 
during  the  fertilization  process  which  was  the  beginning  of  the 
new  individual,  whose  cells  are  now  preparing  for  reproduction; 
they  have  remained  separate  throughout  the  life  of  this  organ- 
ism until  this  event,  through  all  the  divisions  of  the  ancestral 
germ-forming  cells  (Fig.  80).  In  a  certain  sense,  therefore,  this 
process  of  synapsis  represents  the  real  climax  of  the  whole 

splitting  is  less  complete.  One  ring  is  cut  through  showing  two  free  ends.  F. 
Advanced  synizesis.  G.  Chromosomes  appearing  after  synizesis,  shortened  and 
thickened.  The  ex-conjugant  chromosomes  (univalent)  have  separated  and  show 
transverse  constrictions,  preparatory  to  the  second  maturation  division. 


136  GENERAL  EMBRYOLOGY 

series  of  developmental  processes,  and  it  is  at  the  same  time  the 
starting  point  of  the  life  cycle  of  a  new  organism  of  another 
generation. 

In  a  few  forms  (e.g.,  some  Insects,  Lepidosiren,  Fig.  72),  the 
double  nature  of  these  bivalent  chromosomes  is  distinctly  visible 
and  is  indicated  by  a  split  through  the  long  axis  of  the  chromo- 
some, showing  that  the  pair  of  univalent  elements  have  fused 
side  by  side,  a  condition  known  as  parasynapsis.  In  most 
cases  observed  (other  Insects,  Amphibia)  the  fusion  is  end  to 
end,  a  condition  known  as  telosynapsis  (Wilson's  terms).  In 
many  instances,  however,  the  fusion  seems  to  have  occurred 
between  the  granules  composing  the  chromosomes,  so  that  in 
the  bivalent  body  there  is  no  visible  indication  of  the  duplex 
nature  at  this  time;  this  is  then  only  to  be  inferred  from  the 
fact  of  numerical  reduction.  It  is  very  important  to  notice 
that  the  time  relations  between  synizesis  and  synapsis  may 
sometimes  be  just  the  reverse  of  that  described  above,  and  the 
synapsis  stage  may  occur  first,  so  that  the  numerical  reduction 
of  the  chromosomes  occurs  at  the  close  of  the  last  spermatogo- 
nial  division  (some  plants,  Strasburger,  Overton). 

Following  the  period  of  synapsis  the  nucleus  and  cell  may 
proceed  at  once  to  divide,  or  there  may  ensue  another  resting 
period,  during  which  the  chromosomes  again  become  indistinct. 
In  either  case,  when  the  new  mitotic  figure  forms,  always  after 
an  unusually  long  prophase  which  is  characteristic  of  this  divi- 
sion, the  reduced  (haploid)  and  bivalent  chromosomes  often 
show  an  unusual  condition,  in  that  they  may  prepare  at  once, 
not  for  a  single  ensuing  division,  but  for  two  divisions  which  are 
to  follow  immediately,  without  an  intervening  resting  period. 

From  this  stage  onward  in  the  history  of  the  sperm  and  egg 
nuclei,  two  general  types  of  chromosome  behavior  are  some- 
times distinguished,  although  they  are  connected  by  transi- 
tional conditions  and  so  are  regarded  as  modifications  of  a 
single  process.  As  one  extreme  condition  we  find  a  form  of 
chromosome  behavior  called  tetrad  formation,  which  we  may 
describe,  not  because  it  is  a  typical  method  of  chromosome 
reduction,  but  because  the  facts  of  reduction  come  out  more 


MATURATION 


137 


clearly  in  this  form  of  maturation  division  (Boveri).  In  cases 
of  tetrad  formation,  when  the  chromosomes  appear  in  the 
primary  spermatocyte,  after  the  resting  stage,  each  of  the  newly 
organized  bivalent  elements  comes  out  in  the  form  of  four 
small  bodies,  the  tetrads,  arranged  approximately  in  a  square 
(Fig.  73,  E).  These  bodies  result  from  two  successive  splittings 
of  each  chromosome  into  four  columns  of  granules,  each  of 


FIG.  73. — Tetrad  formation  in  the  spermatogenesis  of  Ascaris  megalocephala 
bivalens.  After  Brauer.  X  795.  A-G.  Stages  in  the  division  of  the  primary 
spermatocyte.  A,  B,  splitting,  and  C,  condensation  of  chromatin  thread,  seen 
in  side  view.  D  shows,  in  end  view,  that  the  splitting  is  double.  Centrosome 
divided.  E.  Migration  of  centrosomes  and  formation  of  spindle.  F,  G.  Separa- 
tion of  the  two  groups  of  dyads  and  division  of  the  cell  body.  H.  Secondary 
spermatocyte  containing  two  dyads.  /.  Division  of  secondary  spermatocyte. 
J.  Two  of  the  spermatids,  each  with  two  "monads"  or  single,  univalent,  chromo- 
somes. 

which  is  then  condensed  into  a  single  element  (Fig.  73,  A,  B,  C, 
D).  We  may  recognize  here  a  precocious  division  of  the  chro- 
mosomes, which  in  these  cases  precedes  considerably  the 
division  of  the  nucleus  and  cell  as  a  whole.  Not  only  this,  but 
there  are  two  chromosomal  divisions,  corresponding  with  two 


138  GENERAL  EMBRYOLOGY 

cell  divisions,  and  these  occur  simultaneously,  while  the 
nuclear  and  cytoplasmic  divisions  occur  consecutively  at  a 
later  period,  during  which  these  chromosomes  do  not  divide 

again.     The  number  of  tetrad  groups  is  thus  the  same  as  the 

/s\ 
haploid  number  of  chromosomes  1^1,  and  the  total  number  of 

elements  composing  the  tetrads  is  four  times  the  haploid  or 
two  times  the  diploid  number  (2s). 

The  nucleus  and  cell  now  enter  upon  a  mitosis  in  which  each 
tetrad  behaves  as  a  typical  chromosome.  The  division  and 
migration  of  the  centrosomes  to  opposite  sides  of  the  nucleus, 
the  formation  of  the  spindle  and  asters,  and  other  details  of 
this  mitosis,  have  no  unusual  features  and  need  not  detain  us. 
The  tetrads,  containing  all  told  2s  elements,  become  arranged 
about  the  equator  of  the  spindle  and  each  separates  into  two 
pairs  of  elements  called  the  dyads  (Fig.  73,  F,  G).  The  groups 
of  dyads  then  move  to  opposite  poles  of  the  spindle  and  the  cell 
divides  into  the  two  secondary  spermatocytes.  Since  the  rest- 
ing stage  is  now  omitted,  the  dyads  do  not  dissolve  after  this 
division,  nor  do  they  divide  again  in  anticipation  of  the  next 
mitosis — the  division  of  the  chromatic  elements  for  this  cell 
division  has  already  occurred  in  the  nucleus  of  the  primary 
spermatocyte,  as  we  have  seen.  The  dyads,  containing  all  told 
s  elements,  then  move  at  once  to  the  equator  of  the  new  spindle, 
and  each  separates  into  two  monads  (Fig.  73,  I,  J).  The  two 

o 

groups  of  monads,  each  now  containing  -  elements,  diverge  to 

opposite  poles  of  the  spindle,  and  the  division  of  the  cell 
(secondary  spermatocyte)  results  in  the  formation  of  two  sper- 

o 

matids,  each  with  ^  chromosomes    (Fig.    74).     Each  nucleus 

then  reforms  into  a  typical  resting  condition,  and  passes  through 
the  metamorphosis  into  the  head  of  the  spermatozoon,  as 
described  in  the  preceding  chapter.  The  essential  characteris- 
tic, therefore,  of  the  nucleus  of  the  spermatid  and  spermatozoon 
is  that,  while  each  of  the  bivalent  chromosomes  of  the  spermatocyte 
is  represented,  yet,  as  the  result  of  the  process  of  reduction 


MATURATION 


139 


through  synapsis,  the  chromosomes  are  now  only  half  as  numer- 
ous as  in  all  the  other  cells  of  the  organism.  But  while  each 
bivalent  chromosome  is  thus  represented,  it  may  be  a  question 


FIG.  74.— Diagram  of  reduction  with  tetrad  formation  in  sperm atogenesis. 
From  Wilson,  "Cell."  The  somatic  number  of  chromosomes  is  supposed  to  be 
four.  A,B.  Division  of  one  of  the  spermatogonia,  showing  the  full  number 
(four)  of  chromosomes.  C.  Primary  spermatocyte  preparing  for  division;  the 
chromatin  forms  two  tetrads.  D,E,F.  First  division  to  form  two  secondary 
spermatocytes  each  of  which  receives  two  dyads.  G,H.  Division  of  the  two 
secondary  spermatocytes  to  form  four  spermatids.  Each  of  the  latter  receives 
two  single  chromosomes  and  a  centrosome  which  persists  in  the  middle-piece  of 
the  spermatozoon. 

whether  each  univalent  chromosome  is  also  somehow  repre- 
sented.    To  this  we  shall  return  later. 

But  the  formation  of  tetrads  is  by  no  means  of  universal, 
even  of  common,  occurrence  in  reducing  divisions.  Tetrads  are 
commonly  found  only  among  the  Nematodes,  Annelids,  and 


140 


GENERAL  EMBRYOLOGY 


Arthropods.  In  the  great  majority  of  animals  the  reduction 
divisions  proceed  without  the  formation  of  actual  tetrads  in 
their  typical  form,  and  when  the  bivalent  chromosomes  appear 
in  the  nuclei  of  the  primary  spermatocytes,  as  the  result  of 
synapsis,  they  have  no  unusual  form.  As  they  pass  into  the 
equatorial  plate,  however,  it  is  seen  that  the  two  longitudinal 
halves,  composing  each  bivalent  chromosome,  are  united  at 
their  ends  (Figs.  72,  E;75).  When  the  anaphase  begins  each 
chromosome-half  is  drawn  out  first  from  its  middle,  so  that  the 


FIG.  75. — Maturation  divisions  in  certain  Insects,  showing  forms  of  chromo- 
somes and  their  relation  to  tetrads.  After  de  Sinety.  X  1125.  A,B.  Two 
stages  in  anaphase  of  primary  spermatocyte  division  in  Stenobothrus  parallelus. 
Rings  opening  into  Vs,  which  diverge.  C.  Anaphase  of  spermatogonial  division 
in  Orphania  denticauda,  showing  differentiated  chromosome,  x.  D,  E.  Prepara- 
tion for  first  spermatocyte  division  in  Orphania,  showing  "tetrads"  in  various 
stages  of  formation  from  rings  and  crosses. 

whole  original  chromosome  may  appear  as  a  ring  or  cross,  or 
some  related  figure.  As  the  halves  separate  each  may  assume 
a  p-  or  >  -form,  the  limbs  of  which  may  come  to  lie  parallel  as 
the  chromosome  approaches  the  pole  of  the  spindle,  and  thus 
may  appear  to  be  double  (Figs.  75,  20).  On  account  of  this 
peculiar  form  assumed  by  the  chromosomes  in  this  division, 
it  is  known  as  the  heterotype  division.  And  it  is  to  be  noted 
that  the  separation  of  the  halves  of  the  bivalent  chromosome 


MATURATION  141 

here,  is  along  the  line  of  a  split  which  is  usually  visible  in  the 
chromosomes  when  they  appear  out  of  the  resting  nucleus. 
As  the  result  of  this  heterotype  division  each  secondary 
spermatocyte  receives  the  haploid  number  of  chromosomes. 
The  second  maturation  division  commonly  shows  none  of  these 
rings  or  crosses,  or  other  figures,  and  is  known  consequently 
as  the  homotype  division  (Flemming's  terms).  The  homotype 
division  of  the  secondary  spermatocyte  follows  the  heterotype 
either  immediately,  or  after  a  considerable  pause,  during  which 
the  chromosomes  sometimes  lose  their  definite  outlines  to  some 
extent.  This  pause,  which  does  not  occur  when  tetrads  are 
formed,  is  probably  related  to  the  fact  that  while  in  the  tetrads 
both  divisions  of  the  chromosomes  occur  at  the  commencement 
of  the  process,  in  this  form  of  reducing  division  the  second  split- 
ting does  not  occur  until  after  its  first  actual  division.  During 
the  homotype  division  the  chromosomes  behave,  then,  essen- 
tially as  in  the  divisions  of  the  usual  type,  and  the  resulting 
spermatids  receive,  just  as  in  tetrad  formation,  the  haploid 
number  of  chromosomes,  just  as  do  the  secondary  spermato- 
cytes.  Numerically,  the  most  important  difference  in  reduc- 
tion with  and  without  tetrad  formation  is  that  in  tetrad  forma- 
tion the  secondary  spermatocytes  have  the  diploid  number, 
and  in  the  absence  of  tetrads,  the  haploid  number  of  chromo- 
somes. This  is  the  result  of  the  fact  that  when  tetrads  are 
formed,  the  division  of  the  chromosomes  actually  belonging  to 
the  secondary  spermatocytes  (second  maturation  division) 
really  occurs  in  the  nucleus  of  the  primary  spermatocyte  (first 
maturation  division),  while  in  the  absence  of  tetrads  the  divi- 
sion of  the  chromosomes  has  the  normal  relation  to  cell  division, 
and  the  haploid  number  persists  from  the  primary  spermatocyte, 
after  synapsis,  to  the  spermatid  and  spermatozoon. 

Before  mentioning  any  further  details  of  chromosome  behav- 
ior during  maturation,  we  must  compare  the  process  of  matura- 
tion as  it  occurs  in  the  ovum  with  that  in  the  sperm.  We  may 
say  at  the  outset  that  in  all  essentials  the  two  histories  are 
identical,  so  that  this  comparison  may  be  brief,  but  there  are  a 
few  differences  to  be  noted. 


142  GENERAL  EMBRYOLOGY 

The  divisions  of  the  oogonia  are  normal ;  the  diploid  number 
of  chromosomes  appear,  and  the  details  of  spindle,  aster,  and 
centrosome,  call  for  no  special  mention.  Aside  from  the 
chromosomal  behavior,  the  divergences  of  the  later  maturation 
divisions  from  the  normal  are  partly  the  result  of  the  enormous 
growth  of  the  egg  cell,  and  partly  in  the  nature  of  adaptation 
toward  ensuring  the  practically  undiminished  size  of  the  ovum 
at  the  end  of  the  process;  that  is,  an  equal  subdivision  of  the 
chromatic  elements  is  accompanied  by  an  unequal  subdivision 
of  the  cytoplasm  and  deutoplasm.  The  general  formation  of 
the  large  primary  oocyte  has  been  described.  We  should 
emphasize  again  the  fact  that  the  maturation  of  this  cell 
frequently  is  not  completed  until  after  the  sperm  cell  has 
actually  entered  its  substance.  If  we  were  describing  the  events 
of  the  maturation  of  the  egg  in  strict  accordance  with  their 
usual,  though  not  invariable,  time  relations  we  should  next 
describe  the  ensemination  of  the  egg — the  first  step  in  fertiliza- 
tion. For  the  sake  of  clearness,  however,  we  shall  describe 
maturation  as  if  it  occurred  before  the  entrance  of  the  sperm; 
as  a  matter  of  fact,  there  are  a  few  forms  in  which  this  is  really 
the  normal  course  of  events,  as  in  the  sea-urchin  and  most 
Echinoderms. 

The  nucleus  of  the  oogonium  is  very  large  and  lies  toward  one 
side  of  the  cell — practically  always  toward  the  animal  pole  of  the 
egg  (Fig.  76,  A).  The  first  steps  in  the  maturation  of  the  ovum 
closely  resemble  those  in  the  sperm.  During  the  brief  synizesis 
stage  the  chromatin  condenses  near  the  centrosome,  closely 
around  the  large  "nucleolus"  which  is  commonly  found  in  most 
oocytes;  the  emerging  spireme  shows  that  synapsis  has  occurred 
for  the  spireme  is  segmented  into  the  haploid  number  of  ele- 
ments, representing  the  bivalent  chromosomes.  That  is,  the 
actual  reduction  occurs  in  the  primary  oocyte  as  in  the  primary 
spermatocyte.  The  oocyte  nucleus  then  passes  through  a 
condition  not  represented  in  the  spermatocyte  in  that  a  large 
amount  of  chromatin  leaves  the  chromosomes  (spireme  seg- 
ments), either  dissolving  in  the  nuclear  sap,  or  passing  in  the 
form  of  granules  or  small  masses  to  some  region  of  the  nucleus 


MATURATION 


143 


quite  apart  from  the  chromosomes  proper  (Fig.  76).  It  is 
important  to  note  that  no  chromosomes  are  lost  in  this  way; 
the  full  haploid  number  of  these  bodies  remains  grouped  at  one, 
usually  the  distal,  side  of  the  nucleus.  During  the  active 
preparation  for  the  first  maturation  or  heterotype  division 
the  nuclear  membrane  disappears,  liberating  the  dissolved 


FIG.  76. — Maturation  in  the  egg  of  the  Nemertean,  Cerebratulus.  After  Coe. 
C,  D,  X  375,  others  X  250.  A.  Primary  oocyte.  Part  of  the  chromatin  has 
been  condensed  into  chromosomes,  only  five  of  which  are  shown  (the  number 
present  is  sixteen).  The  remainder  of  the  chromatin  is  thrown  out  into  the  cyto- 
plasm. The  centrosomes,  each  with  a  small  aster,  are  diverging,  and  the  nuclear 
membrane  is  commencing  to  disappear.  B.  First  polar  spindle  fully  formed  and 
rotated  into  radial  position.  Chromosomes  in  equatorial  plate.  C.  First 
oocyte  division;  anaphase.  D.  First  polar  body  nearly  separated.  E.  First 
polar  body  completely  cut  off;  second  polar  spindle  formed  and  rotating  into 
radial  position.  Spermatozoon  within  the  egg.  F.  Second  polar  body  com- 
pletely separated.  Egg  pronucleus  forming,  surrounded  by  large  aster.  Sperm 
pronucleus,  also  with  a  large  aster,  enlarged  and  approaching  the  egg  pro- 
nucleus,  c,  chromosomes;  o,  nucleolus,  vacuolated  and  commencing  to  disap- 
pear; s,  spermatozoon  just  within  the  egg;z\  germinal  vesicle;  vc,  contents  (extra 
chromosomal)  of  germinal  vesicle.  /,  //,  first  and  second  polar  bodies;  d\  sperm 
pronucleus;  Q  ,  egg  pronucleus. 

chromatin,  or  the  extra-chromosomal  masses,  which  then 
disappear  gradually  into  the  cytoplasm  while  the  small  chromo- 
somes pass  into  the  division  figure.  Sometimes  this  loss  of 
chromatin  is  effected  by  a  shrinkage  of  the  chromosomes  them- 


144  GENERAL  EMBRYOLOGY 

selves,  as  in  some  Elasmobranchs,  where  the  chromosomes  at  this 
time  shrink  to  about  one-fortieth  of  their  previous  length  and 
one-tenth  their  previous  diameter  (Riickert,  Fig.  34).  Or  the 
"nucleolus"  of  the  oocyte  may  be  a  karyosome  or  chromatin 
nucleolus,  and  in  such  cases  (Echinoderms,  for  example)  during 
synizesis  the  chromatin  may  be  nearly  all  contained  within  this 
body.  Then  the  chromosomes  are  formed  singly  or  in  groups 
out  of  this  "  chromatin  reservoir."  After  they  have  all  been 
given  off,  much  the  greater  part  of  the  chromatin  still  remains 
in  the  karyosome,  which  then  may  fragment  before  dissolu- 
tion, or  it  may  be  dissolved  directly  (Fig.  35).  The  subsequent 
behavior  of  the  chromosomes  is  closely  similar  to  that  of  the 
spermatocyte  chromosomes;  tetrads  may  or  may  not  be 
formed,  according  to  the  species,  as  the  chromosomes  pass  into 
the  division  figure  (Figs.  77,  78).  The  centrosome  divides  and 
the  spindle  and  asters  form  typically  in  most  respects  save  in 
size  and  position.  The  spindle  is  very  small  and  in  most  eggs 
is  close  to  the  surface  of  the  cell  at  its  animal  pole  (Fig.  76).  In 
alecithal  and  isolecithal  eggs  the  nucleus  and  spindle  are  at 
first  located  centrally  and  then  later  move  to  the  periphery. 
At  first  the  spindle  lies  parallel  to  a  tangential  plane,  but  during 
the  mesophase  it  rotates  through  ninety  degrees,  putting  its 
axis  in  a  radial  direction  (Figs.  76,  77).  In  many  cases  the 
division  of  the  oocyte  is  inhibited  at  this  stage,  until  after  the 
entrance  of  the  spermatozoon,  when  it  proceeds  to  completion; 
or  this  heterotype  division  may  proceed  without  any  inter- 
ruption and  the  primary  oocyte  cut  at  once  into  two  cells.  The 
extremely  eccentric  position  of  the  nucleus  in  this  stage  leads  to 
one  of  the  most  characteristic  features  of  oogenesis,  namely,  the 
very  unequal  division  of  the  cell  body.  One  of  the  products  of 
division,  the  secondary  oocyte,  is  of  practically  the  same  size  as 
the  primary  oocyte;  the  other  cell — the  first  polar  body — is 
very  much  smaller,  indeed  usually  minute  (Figs.  76,  77,  78). 
In  essentials  these  two  daughters  of  the  primary  oocyte  are 
equivalent;  their  nuclei  are  alike  in  size  and  composition,  each 
contains  a  daughter  centrosome,  but  with  the  polar  body  there 
is  only  the  smallest  amount  of  cytoplasm  and  practically  none 


MATURATION 


145 


FIG.  77. — Maturation  in  the  egg  of  Ascaris  megalocephala  bivalens.  From 
Wilson,  "Cell,"  after  Boveri.  A.  Egg  with  spermatozoon  just  entering  at  d\ 
The  germinal  vesicle  contains  two  rod-shaped  tetrads  (only  one  clearly  shown). 
B,  C.  Tetrads  seen  in  profile  and  end  views.  D.  First  polar  spindle  forming  (in 
this  case  within  the  germinal  vesicle).  E.  First  polar  spindle  in  definitive  posi- 
tion. F.  Tetrads  dividing.  G.  First  polar  body  formed,  containing,  like  the 
egg,  two  dyads.  H,  I.  Dyads  rotating  into  position  for  the  second  division. 
«/.  Dyads  dividing.  K.  Each  dyad  has  divided  into  two  single  chromosomes, 
as  the  second  polar  division  approaches.  (For  final  stages,  see  Fig.  94.) 


146 


GENERAL  EMBRYOLOGY 


H 

FIG.  78. — Diagram  of  reduction,  with  tetrad  formation,  in  oogenesis.  From 
"Wilson,  "Cell."  The  somatic  number  of  chromosomes  is  supposed  to  be  four. 
A.  Initial  phase;  two  tetrads  have  been  formed  in  the  germinal  vesicle.  B.  The 
two  tetrads  have  been  drawn  up  about  the  spindle  to  form  the  equatorial  plate 
of  the  first  polar  mitotic  figure.  C.  The  mitotic  figure  has  rotated  into  position, 
leaving  the  remains  of  the  germinal  vesicle  at  g.v.  D.  Formation  of  the  first 
polar  body;  each  tetrad  divides  into  two  dyads.  E.  First  polar  body  formed; 
two  dyads  in  it  and  in  the  egg.  F.  Preparation  for  the  second  division.  G. 
Second  polar  body  forming  and  the  first  dividing;  each  dyad  divides  into  two 
single  chromosomes.  H.  Final  result;  three  polar  bodies  and  the  egg-nucleus 
(9),  each  containing  two  single  chromosomes  (half  the  somatic  number);  c,  the 
egg-centrosome  which  now  degenerates  and  is  lost. 


MATURATION  147 

of  the  deutoplasmic  substance.  The  equal  division  of  the 
nucleus  is  thus  accomplished  without  appreciable  loss  from  the 
oocyte  of  any  of  the  cytoplasm  and  food  reserve. 

The  second  maturation  or  homotype  division  follows,  either 
at  once,  or  after  a  long  pause  in  cases  where  the  sperm  normally 
enters  at  this  time.  The  figure  for  the  second  maturation 
division  is  either  a  new  figure  or  the  reorganized  remains  of  the 
preceding.  In  either  case  it  appears  in  the  region  occupied  by 
its  predecessor,  often  in  a  radial  position  from  its  first  appear- 
ance (Figs.  76,  77).  Again  the  division  is  very  unequal  and  the 
secondary  oocyte  gives  rise  to  the  large  mature  ovum  and  a 
small  second  polar  body,  again  alike  as  regards  nuclear  com- 
position. The  first  polar  body  may  or  may  not  divide  into 
two  at  the  same  time;  we  may  assume  that  such  a  division  is 
normal,  but  on  account  of  the  degeneration  of  the  polar  bodies 
such  a  division  tends  to  disappear;  in  different  forms  various 
stages  of  this  division  are  reached.  In  a  few  rare  instances, 
as  in  some  Rotifers  and  Insects,  the  second  polar  body  may  also 
divide. 

The  chromosomes  remaining  in  the  ovum  then  re-form  a 
reticular  nucleus,  smaller  than  the  original  oocyte  nucleus,  and 
with  the  haploid  number  of  chromosomes;  after  forming  a  thin 
membrane  the  nucleus  moves  toward  the  cytoplasmic  center 
of  the  ovum  and  there  awaits  union  with  the  nucleus  of  the 
fertilizing  spermatozoon  (Figs.  76,  77).  With  few  if  any  ex- 
ceptions, the  centrosome  of  the  ovum  is  entirely  lost  as  the 
nucleus  is  reconstructed;  the  absence  of  the  centrosome  is  one 
of  the  peculiarities  of  the  egg  cell.  It  should  be  added  that 
when  the  maturation  is  not  completed  until  after  the  entrance 
of  the  sperm,  the  egg  does  not  ordinarily  re-form  a  typical 
nucleus  but  proceeds  at  once  to  unite  with  the  sperm  nucleus. 

The  final  result  of  the  two  maturation  divisions  of  the 
primary  oocyte  is  the  formation  of  four  cells  whose  nuclei  are 
similar,  and  which  are  morphologically  exactly  equivalent  to 
one  another  (Figs.  74,  78).  Physiologically,  however,  there  is 
the  greatest  difference  among  them,  since  of  the  four  only  one, 
the  ovum,  is  able  to  function,  or  indeed  even  to  remain  alive, 


148 


GENERAL  EMBRYOLOGY 


and  share  in  the  development  of  a  new  organism.  The  other 
three  (or  two),  the  polar  bodies,  after  a  brief  time  degenerate 
and  disappear. 

The  view  that  the  ovum  and  the  polar  bodies  are  equivalent 
cells  morphologically,  and  that  the  latter  are  in  reality  to  be 
looked  upon  as  degenerate  egg  cells  (Mark)  is  now  familiar. 
Their  identity  in  nuclear  structure  and  history  is  of  course  a 
decisive  similarity. 


FIG.  79. — Variations  in  the  size  of  polar  bodies.  A,  B.  Sections  through 
segmenting  ovum  of  the  Gasteropod,  Limax  maximus,  showing  polar  bodies  of 
very  different  sizes.  After  Meisenheimer.  C-F.  Ascaris  megalocephala.  C,  D, 
E,  after  Sala,  show  influence  of  low  temperature;  F,  after  Boveri.  C.  Large 
first  polar  body  which  has  divided.  D.  Very  large  first  polar  body.  E.  Equal 
division  of  egg  during  "first  polar  division."  F.  Equal  division  of  egg  during 
"second  polar  division."  /,  //.  First  and  second  polar  bodies;  9,  egg  pro- 
nucleus;  d\  sperm  pronucleus. 

The  actual  size  of  the  polar  bodies  varies  enormously.  Those 
of  the  Echinoderms  are  among  the  smallest,  some  being  only 
5-8  micro,  (1/5000-1/3000  inch)  in  diameter;  in  Amphioxus 
they  are  about  7  micro,  or  about  one-fourteenth  the  diameter 
of  the  egg,  while  in  the  mouse  they  are  relatively  much  larger — 
13  micro,  or  one-fifth  the  diameter  of  the  egg.  An  interesting 
series  of  forms  can  be  arranged  bridging  over  the  size  differences, 
and  to  some  extent  also  the  physiological  differences  between 
the  ovum  and  the  polar  bodies.  In  a  few  Annelids,  some 


MATURATION  149 

Turbellaria,  and  most  Molluscs  (Fig.  79)  the  polar  bodies  are 
very  large,  sometimes  even  one-fourth  the  diameter  of  the 
egg  itself.  In  occasional  instances,  abnormal  " giant"  polar 
bodies  are  formed,  which  really  approach  the  ovum  in  size 
(e.g.,  Amphioxus,  and  the  Turbellarian,  Prosthecerceus) .  Some 
of  these  large  polar  bodies  form  a  definite  membrane,  like  the 
vitelline  membrane  of  the  ovum;  and  in  rare  instances  they  are 
actually  fertilizable,  although  their  development  never  proceeds 
beyond  an  incomplete  cleavage.  The  last  step  in  the  transition 
from  polar  body  to  egg  cell  is  represented  by  an  interesting 
condition  occasionally  found  in  Ascaris  and  some  other  forms, 
(e.g.,  mouse)  where  an  abnormally  placed  polar  spindle  may 
result  in  the  division  of  the  oocyte  into  two  equal  cells,  one  of 
which  should  be  called  a  polar  body  (Fig.  79,  E,  F).  The  last 
step  in  the  opposite  direction — the  dissimilarity  between  egg 
and  polar  body — reaches  a  climax  in  some  of  the  Insects  where 
polar  bodies  are  not  really  formed  as  such;  the  oocyte  nucleus 
divides  as  in  polar  body  formation  and  daughter  nuclei  are 
formed  but  these  remain  in  the  periphery  of  the  egg  cell,  for 
no  cytoplasmic  division  whatever  is  accomplished.  The 
nuclear  phase  is  the  only  part  of  the  maturation  division 
remaining.  The  "polar  nuclei"  formed  in  this  way  degenerate 
without  sharing  in  development,  just  as  if  they  had  been  cast 
out  of  the  cell. 

We  have  already  suggested  that  this  dissimilarity  in  size 
between  the  polar  bodies  and  the  ovum  is  in  the  nature  of  an 
adaptation  such  that  maturation  of  the  egg  nucleus  may  take 
place  without  reducing  the  amount  of  cytoplasm  and  food 
materials  which  are  to  form  the  chief  portion  of  the 
material  basis  of  the  new  organism.  In  accomplishing  this, 
three  of  every  four  potential  egg  cells  are  totally  deprived  of 
these  substances  and  lose  all  possibility  of  developing.  In 
some  few  instances,  the  polar  bodies  may  for  a  time  remain 
alive  and  during  the  early  cleavage  show  some  signs  of  activity, 
such  as  the  performance  of  amoeboid  movement,  "spinning 
activity,"  etc.  (Andrews). 

The  location  of  the  polar  bodies  within  or  without  the  vitel- 


150  GENERAL  EMBRYOLOGY 

line  membrane  depends  upon  the  time  relation  between  mem- 
brane formation  and  maturation.  The  polar  bodies  may 
form  before  the  membrane,  in  which  case  they  usually  are  lost 
from  the  egg  soon  after  their  formation.  More  frequently 
they  form  after,  and  therefore  within,  the  membrane  and  so 
can  be  seen  for  some  time  after  development  begins,  when 
they  form  useful  points  for  orientation,  for  in  nearly  all  cases 
they  mark  the  animal  pole  of  the  egg.  The  only  exceptions  to 
this  location  are  among  the  Insects  and  Copepods,  in  which 
then*  position  is  variable. 

Morphologically  there  is  also  complete  correspondence 
between  the  ovum  and  the  three  polar  bodies  formed  from  the 
primary  oocyte,  and  the  four  spermatids  and  spermatozoa 
formed  from  the  primary  spermatocyte  (Platner,  O.Hertwig). 
But  again  there  is  physiological  divergence,  in  that  all  of  the 
derivatives  of  the  spermatocyte  are  capable  of  functioning  as 
germ  cells,  while  only  one  of  the  oocyte  derivatives  may  do  so. 
This  physiological  divergence  is  an  expression,  in  another  form, 
of  the  physiological  division  of  labor  between  the  egg  and  the 
sperm  already  referred  to. 

The  chief  points  of  similarity  and  difference  between  ovum 
and  spermatozoon  are  summarized  in  the  accompanying  table. 

Little  is  known  regarding  the  nature  of  the  stimuli  which 
lead  to  the  process  of  maturation,  but  it  is  clear  that  they  are 
quite  varied  in  different  eggs.  In  some  of  those  cases  where  the 
eggs  are  discharged  freely  into  the  water,  contact  with  the 
water  seems  to  initiate  the  process.  But  maturation  may  be 
begun  previously  to  such  a  discharge.  In  some  cases  of  this 
kind,  as  well  as  in  others  where  the  eggs  are  not  thus  freed,  the 
rupture  of  the  egg  follicle  seems  to  start  the  maturation  process. 
In  many  cases  the  entrance  of  the  sperm  into  the  ovum  is  the 
effective  stimulus  to  maturation,  or  to  the  completion  of 
maturation  in  many  of  those  instances  where  it  has  been  begun 
previously  and  has  been  inhibited,  either  just  before  or  just 
after  the  formation  of  the  first  polar  body. 

While  we  must  postpone  a  part  of  our  brief  discussion  of  the 
theoretical  significance  of  maturation,  for  reasons  stated 


MATURATION 


151 


COMPARISON  OF  TYPICAL  OVUM  AND  SPERMATOZOON 
Similarities: 

Nuclei  contain  the  haploid  number  of  chromosomes,  the  result  of 

two  similar  meiotic  divisions. 
Chromosomes  alike  in  form,  size,  and,  with  a  few  exceptions  of 

special  significance,  in  number. 
Can  function  only  after  syngamy. 


Differences: 

SPERMATOZOON 

Little  cytoplasm. 
No  deutoplasm. 

Actively  motile. 

Centrosome  present. 

One  of  four  similar  products 
of  the  division  of  the  sper- 
matocyte,  all  of  which  are 
functional. 

Usually  completely  formed  and 
matured  in  the  gonad. 


OVUM 

Much  cytoplasm. 

Nearly  always  contains  deuto- 
plasm, often  very  large  amounts. 

Non-motile. 

Centrosome  absent. 

The  functional  one  of  four  dis- 
similar products  of  the  division 
of  the  oocyte,  the  other  three 
of  which  are  alike  and  not  func- 
tional. 

Usually  formed  but  rarely  com- 
pletely matured  in  the  gonad. 


above,  we  should  indicate  at  this  time  that  the  process  has 
significance  from  at  least  two  points  of  view.  As  a  preliminary 
we  should  note  that  two  different  processes  are  involved  in 
maturation;  first,  a  reduction  in  the  number  of  chromosomes, 
second,  a  reduction  in  the  amount  of  chromatin.  The  earlier 
idea  that  maturation  is  merely  a  process  by  which  the  germ 
cells  are  rid  of  a  part  of  then*  chromatin,  and  one-half  of  their 
chromosomes,  as  a  preparation  for  the  restoring  union  of 
chromatin  and  chromosomes  during  fertilization,  is  only  one 
and  probably  the  least  important  aspect.  Many  cells  other 
than  germ  cells  gain  and  lose  large  amounts  of  chromatin,  and 
without  going  through  any  such  complex  process  as  that  out- 
lined above.  Frequently  much  more  than  half,  sometimes 
fully  nine-tenths,  of  the  chromatin  is  lost  from  the  oocyte 
nucleus  during  maturation,  while  during  spermatogenesis  com- 
paratively little  may  be  lost.  And  the  mere  numerical  reduc- 


152  GENERAL  EMBRYOLOGY 

tion  of  chromosomes  is  fully  accomplished  in  synapsis,  before 
the  actual  maturation  divisions  occur.  For  these  and  many 
other  reasons  it  seems  that  the  chief  importance  of  maturation 
is  from  the  standpoint  of  inheritance.  This  is  true  particularly 
of  most  of  the  details  regarding  chromosome  reduction,  which 
become  significant  only  when  correlated  with  the  facts,  first, 
that  the  germ  cells  are  the  simplest  phases  in  the  life  cycle  of 
the  organism,  alternating  with  the  mature  phases,  the  complex 
characteristics  of  which  are  related  to  the  simpler  characters 
of  the  germ,  and  second,  that  in  some  way,  as  yet  unknown, 
the  structural  and  physiological  characteristics  of  the  new 
organism  are,  at  least  in  part,  primarily  determined  by  the 
chromosomal  structure  of  both  of  the  germ  nuclei,  i.e.,  to  the 
fact  of  biparental  inheritance.  From  the  standpoint  of  in- 
heritance then  the  details  of  the  behavior  of  the  chromosomes 
during  the  maturation  divisions  take  on  the  greatest  importance. 
One  of  the  more  important  matters  is  the  precise  plane  of 
division  of  the  chromosomes.  It  seems  necessary  to  assume 
that  each  chromosome  is  not  entirely  homogeneous,  but  that 
its  qualities  differ  in  different  parts.  Consequently,  in  chromo- 
somal composition  the  four  nuclei  derived  from  each  primary 
oocyte  or  spermatocyte  nucleus  may  be  all  alike  or  may  be  of 
different  kinds,  according  to  whether  the  original  chromosomes 
separated  into  similar  or  dissimilar  parts  in  one  or  both  of  the 
maturation  divisions  (Fig.  80) .  In  those  cases  where  the  chromo- 
somes separate  into  qualitatively  similar  halves  the  division 
is  said  to  be  equational,  and  when  into  qualitatively  unlike 
halves  the  division  is  reducing.  And  it  is  commonly  believed 
that  one  of  the  maturation  divisions  is  equational  and  one  re- 
ducing. When  the  equational  division  precedes,  post- 
reduction  is  said  to  occur;  when  the  reducing  division  precedes 
it  is  described  as  prereduction  (Korschelt  and  Heider).  It  is 
by  no  means  a  simple  matter  to  determine  whether  a  given 
chromosome  division  is  equational  or  reducing,  since  there  is 
externally  visible  no  indication,  in  a  chromosome  itself,  of  its 
qualitative  differentiation;  and  further  because  the  processes 
of  rearrangement  and  redistribution  of  the  chromatin  granules 


MATURATION  153 

making  up  the  chromosome  are  usually  very  obscure,  particu- 
larly when  synizesis  is  pronounced. 

This  whole  subject  is  in  a  rather  more  hypothetical  state 
than  one  might  wish,  considering  the  importance  of  the  con- 
clusions to  be  drawn.  Upon  the  assumption  that  the  qualities 
of  a  chromosome  differ  from  end  to  end,  a  longitudinal  fission 
of  the  chromosome  would  divide  it  into  exactly  similar  halves, 
while  a  transverse  fission  would  of  course  divide  it  dissimilarly. 
In  many  forms  it  is  clear  that  the  first  or  heterotype  division 
is  longitudinal  and  the  second  or  homotype  division  is  trans- 
verse so  that  the  four  resulting  nuclei  are  of  two  categories.  In 
other  cases  it  appears  superficially  that  both  divisions  are 
longitudinal,  while  in  still  others  it  is  really  impossible  to  say 
definitely  whether  a  given  division  is  longitudinal  or  transverse. 
A  transverse  division  would  be  reducing,  however,  only  upon 
the  assumption  of  an  end  to  end  differentiation  of  the  chromo- 
some, and  upon  the  further  assumption,  which  should  be  clearly 
apprehended,  that  no  rearrangement  of  the  chromatin  granules 
occurs  during  the  maturation  divisions.  And  the  additional 
fact  must  be  taken  into  consideration  that  the  chromosomes 
of  the  primary  spermatocyte  or  oocyte  are  bivalent,  that  they 
represent  two  chromosomes,  which  as  wholes  have  fused  in 
either  parasynapsis  or  telosynapsis.  and  the  actual  chromatin 
granules  composing  them  may  have  an  arrangement  in  fusion 
which  is  not  indicated  by  the  behavior  of  the  whole  chromosome. 
The  question  whether  a  given  longitudinal  or  transverse 
division  of  a  chromosome  is  equational  or  reducing  then  can  be 
determined  only  by  taking  all  of  these  preliminary  arrange- 
ments into  account,  and  in  most  cases  this  is  extremely  difficult 
or  even  at  present  impossible.  The  small  size  of  the  chromo- 
somes themselves  and  the  minuteness,  often  invisibility,  of  the 
chromatin  granules,  often  put  the  facts  of  their  •arrangement 
and  behavior  beyond  the  possibility  of  observation,  and  we 
can  only  infer  their  arrangement  and  history  from  subsequent 
events.  In  most  cases  we  know  only  that  reduction  occurs. 
And  from  the  few  cases  in  which  the  course  of  events  seems 
clear,  we  infer  that  in  all  maturation  divisions,  one  is  equational, 


154 


GENERAL  EMBRYOLOGY 


FIG.  80. — Diagrams  representing  the  behavior  of  the  chromosomes  during 
fertilization  and  maturation.  The  differentiation  of  the  three  kinds  of  chromo- 
somes is  indicated  by  the  number  of  small  circles  in  each.  6\  chromosomes 
derived  from  the  spermatozoon  (sperm  pronucleus)  (black  circles) ;  9  ,  those  from 
the  egg  (egg  pronucleus)  (white  circles).  The  somatic  number  of_chromosomes 
is  six.  A.  Entrance  of  spermatozoon.  B.  Fusion  of  egg  and  sperm  pronuclei, 
forming  the  first  cleavage  nucleus.  C.  Splitting  of  chromosomes  in  equatorial 
plate,  during  the  division  of  any  somatic,  oogonial,  or  spermatogonial  cell.  D. 
Primary  oocyte  or  spermatocyte  in  synapsis  (telosynapsis).  Fusion  of  similar 
chromosomes  of  maternal  and  paternal  origin.  E.  Longitudinal  splitting  of 
bivalent  chromosomes  during  first  maturation  division.  F.  First  division 
completed  forming  the  two  secondary  oocytes  or  spermatocytes.  The  nuclei 
are  alike  in  composition.  G.  Transverse  division  of  chromosomes  during  second 
maturation  division  (reducing  division — postreduction).  H.  The  resulting 
four  cells.  With  respect  to  each  chromosome  the  cells  are  of  two  kinds,  numer- 
ically equal. 


MATURATION  155 

one  reducing,  resulting  in  the  formation  of  germ  cells  of  two 
more  or  less  unlike  kinds,  in  equal  numbers  (Fig.  80). 

Aside  from  its  relation  to  the  phenomena  of  heredity,  the 
meaning  of  the  maturation  process  is  very  problematic.  In 
the  Protozoa  where  definite  chromosomes  are  formed  only 
infrequently,  maturation  frequently  involves  a  separation 
between  reproductive  and  vegetative  chromatin,  as  already 
suggested.  Among  the  Metazoa  there  is  usually  a  loss  of 
chromatin  from  the  nucleus,  but  it  is  very  doubtful  whether 
it  has  a  similar  meaning.  In  a  great  many  cells,  particularly 
those  which  are  very  active,  e.g.,  gland  cells,  oocytes,  etc., 
the  cytoplasm  is  constantly  receiving  substance  from  the 
nucleus.  This  material  is  frequently  chromatic,  and  the 
granules  of  this  kind  have  received  a  variety  of  special  names, 
but  collectively  may  be  included  under  the  term  chromidia. 
(See  Chapter  II.)  It  may  be  that  through  some  such  process 
as  this  the  nucleus  exercises  those  forms  of  control  and  regula- 
tion of  cell  life  that  are  its  chief  function.  The  loss  and 
degeneration  of  the  chromatin  distributed  to  the  polar  bodies 
can  have  no  significance  here,  for  that  process  is  involved  in  the 
degeneration  of  the  entire  polar  bodies,  which  has  an  entirely 
different  meaning.  But  during  the  growth  period  of  most  ova, 
just  after  synizesis,  a  relatively  large  amount  of  the  chromatin 
is  thrown  out  into  the  cytoplasm,  and  during  the  later  stages 
of  spermatogenesis  a  somewhat  similar  loss  may  be  observed. 
And  in  the  very  early  history  of  the  germ  cells  of  the  organism, 
when  this  may  consist  of  only  a  few  cells,  the  primordial  germ 
cells  may  often  be  distinguished  by  just  this  process  of  chro- 
matin discharge  from  the  nucleus.  Such  cells  are  often 
characterized  by  unusually  large  nuclei,  and  a  large  fraction  of 
their  chromatin  content  may  be  liberated  into  the  cytoplasm 
at  each  mitosis.  It  may  very  well  be,  therefore,  that  this  is  a 
regular  and  highly  significant  process  in  the  formation  and 
maturation  of  the  germ  cells,  having  to  do  with  the  unusual 
activity  of  the  sperm  or  with  the  development  of  various  formed 
substances,  both  protoplasmic  and  deutoplasmic,  present  in 
the  cytoplasm  of  the  ovum.  Indeed  it  may  not  be  too  much 


156 


GENERAL  EMBRYOLOGY 


to  suppose  that  the  all-important  " organization"  of  the  ovum 
may  in  some  way  be  related  to  this  process  of  chromatin 
distribution. 

The  fact  of  maturation  has  been  determined  for  all  groups  of  many- 
celled  animals  and  plants,  and  among  the  unicellular  forms  it  is  by  no 
means  uncommon.  Among  the  Protozoa  the  phenomena  of  maturation 
are  of  considerable  theoretical  interest.  In  those  forms  in  which  the 
chromatin  is  not  formed  into  definite  chromosomes,  but  remains  unor- 


FIG.  81. — Maturation  phenomena  accompanying  conjugation  in  the  Rhizopod, 
Actinophrys  sol.  From  Calkins,  "Protozoa,"  after  Schaudinn.  A.  Parts  of 
two  individuals,  fused;  the  axial  filaments  terminate  in  granules  on  the  surface 
of  the  nucleus.  B.  Nuclei  in  prophase.  C.  Formation  of  first  polar  spindle. 
D.  Reconstruction  of  nuclei.  E.  Fusion  of  nuclei.  F.  First  division  spindle. 
p,  polar  body. 

ganized,  grossly,  there  seems  to  be  a  kind  of  division  of  labor  between 
vegetative  and  kinetic  (reproductive)  forms  of  chromatin.  The  repro- 
ductive nuclei  (idiochromidia)  are  frequently  distinctly  separate  from 
the  somatic  nuclei  (chromidia),  and  just  before  fertilization  the  former 
may  divide  twice  in  rapid  succession.  This  process  bears  the  greater 
resemblance  to  the  maturation  of  the  ovum  since  after  each  division  one 
of  the  two  products  degenerates,  often  without  actually  being  thrown 
out  of  the  cell,  leaving  functional  only  one  of  the  four  products  of  the 
original  idiochromidium.  Actinosphcerium  is  a  typical  example  (R. 


MATURATION  157 

Hertwig) ,  and  there  are  several  other  forms  where  much  the  same  thing 
occurs,  e.g.,  Actinophrys  (Fig.  81),  Entamceba.  In  all  cases  such  as 
these,  it  is  impossible  to  say  whether  reduction,  in  a  strict  sense,  is 
accomplished,  or  whether  this  is  merely  an  elimination  of  chromatin,  for 
the  chromatin  is  not  organized  into  chromosomes  whose  precise  behavior 
may  be  traced ;  it  is  quite  likely  that  there  is  here  no  true  reduction  in 
the  Metazoan  sense.  In  some  of  the  Infusoria,  however,  definite  chro- 
mosomes are  formed  in  the  nucleus  during  these  divisions  and  a  definite 
chromosomal  history  may  be  made  out.  In  Paramcecium,  for  instance, 
as  described  by  Calkins  and  Cull,  where  the  idiochromidia  are  known  as 
the  micronuclei,  these  alone  are  concerned  in  the  "  maturation  "  divisions. 
The  micronucleus  forms  a  fairly  typical  division  figure  consisting 


C 


FIG.  82. — Maturation  divisions  in  Paramcecium  aurelia  (caudatum).  From 
Calkins  and  Cull.  Only  a  few  of  the  chromosomes  are  represented  in  each  case. 
A.  Late  anaphase  of  first  maturation  division  of  micronucleus;  some  chromo- 
somes incompletely  divided.  X  1000.  B.  Early  anaphase  of  second  maturation 
division.  X  633.  C.  Telophase  of  second  maturation  division.  X  900. 

of  a  spindle  and  more  than  200  separate  chromosomes.  During  the 
first  maturation  division  each  of  these  divides  longitudinally,  the 
resultants  passing  in  each  case  to  the  separate  daughter  nuclei,  without 
any  corresponding  division  of  the  cell  body  (Fig.  82) .  A  second  matura- 
tion division  follows  immediately  and  is  precisely  like  the  first  giving 
four  daughter  nuclei  (micronuclei,  idiochromidia),  three  of  which  then 
degenerate  as  in  polar  body  formation.  The  one  remaining  nucleus 
divides  again,  this  time  the  chromosomes  dividing  transversely  (reducing 
division?) .  This  third  division  is  not  strictly  comparable  with  anything 
to  be  found  in  the  Metazoa  and  is  apparently  correlated  with  the  charac- 
ter of  the  fertilization  process  in  this  form,  for  both  parts  share  in  repro- 
duction. One-half  remains  in  situ  as  the  equivalent  (analog)  of  the  egg 
nucleus,  and  the  other  half  migrates,  as  the  equivalent  (analog)  of  the 
sperm  nucleus,  to  the  body  of  another  organism,  fusing  with  (fertilizing) 
the  stationary  nucleus  of  that  individual.  In  the  majority  of  the 
Protozoa  the  so-called  "maturation"  or  "reduction"  divisions  are  not 


158 


GENERAL  EMBRYOLOGY 


equivalent  to  these  processes  in  the  Metazoa,  but  are  merely  divisions 
by  which  a  separation  is  effected  between  the  reproductive  and  nutri- 
tive chromatin,  i.e.,  idiochromatin  and  trophochromatin;  in  nearly  all 
known  forms  only  the  former  takes  any  active  part  in  the  subsequent 
reproductive  processes,  while  the  trophochromatin  usually  dissolves  and 
disappears. 

Two  very  special  modifications  of  the  maturation  process  deserve 
just  a  word.  The  first  is  in  connection  with  those  few  eggs  which 
normally  develop  without  fertilization  (parthenogenesis),  i.e.,  without 
the  union  of  equivalent  egg  and  sperm  nuclei.  In  such  cases,  which  are 


FIG.  83. — Maturation  in  the  parthenogenetic  egg  of  the  brine-shrimp,  Artemia. 
After  Brauer.  A,  X  795,  others,  X  368.  A.  Second  polar  body  incompletely 
cut  off.  B.  Second  polar  nucleus  reentering  the  egg  and  approaching  the  egg 
pronucleus.  C,  D.  Fusion  of  second  polar  body  nucleus  with  egg  pronucleus. 
E.  First  cleavage  spindle  with  two  groups  of  chromosomes  derived  from  the  two 
nuclei.  II.  Second  polar  body  or  nucleus;  $,  egg  pronucleus. 

known  in  the  Aphids,  many  Crustacea,  and  Rotifers,  for  example,  the 
normal  course  of  maturation  would  lead  to  the  formation  of  an  organism 

with  the  haploid  number  of  chromosomes    ( |j  in  all  of  its  cells.     In 

most,  if  not  in  all,  such  cases  which  have  been  studied,  it  is  now  known 
that  as  a  matter  of  fact  the  egg  is  not  left  with  the  reduced  number  of 
chromosomes.  Thus  in  the  brine-shrimp,  Artemia  (Brauer),  which 
illustrates  the  usual  course  of  events  in  parthenogenesis,  the  first  matu- 
ration division  proceeds  as  usual  and  is  equational  (reducing),  leaving 

2  bivalent  chromosomes  in  the  secondary  oocyte  nucleus.  Then  one 
of  two  courses  may  be  followed  (Fig.  83).  In  most  normally  partheno- 


MATURATION 


159 


genetic  eggs  a  second  polar  body  is  formed  typically,  leaving  the  reduced 
number  of  univalent  chromosomes  in  the  egg  nucleus,  but  then  the 
second  polar  body  immediately  reenters  the  egg,  apparently  taking  the 
place  of  an  equivalent  sperm  nucleus  and  restoring  the  chromosomal 
characters  to  the  normal  somatic  condition,  after  which  development 
proceeds.  The  polar  body  need  not  be  actually  extruded  from  the  egg 
cell  in  order  to  give  the  same  history,  as  long  as  the  nuclear  events  are 
equivalent  (Fig.  84).  In  some  eggs,  even  of  species  in  which  the  history 
is  at  times  similar  to  that  just  described,  a  different  method  gives  the 
same  result.  Thus,  while  the  second  polar  spindle  may  form  typically, 
the  chromosomes  upon  it  do  not  divide,  and  the  equivalent  of  the  second 


FIG.  84. — Maturation  in  the  parthenogenetic  egg  of  the  Echinoderm,  Astro- 
pecten.  After  O.  Hertwig.  A.  First  polar  body  formed  but  not  extruded; 
second  polar  division  in  early  anaphase.  B.  First  polar  body  extruded;  second 
polar  division  completed,  the  polar  nucleus  near  the  periphery.  C,  D,  E.  Stages 
in  the  gradual  approach  and  fusion  of  the  second  polar  nucleus  and  egg  pronu- 
cleus,  to  form  the  cleavage  nucleus.  /.  First  polar  body;  //,  second  polar  nu- 
cleus; 9  ,  egg  pronucleus. 

polar  nucleus  is  never  formed.  The  egg  nucleus  then  re-forms  with  its 
-  chromosomes,  but  these  are  bivalent  as  shown  by  the  character  of 

the  first  maturation  division,  so  that  in  effect  the  egg  nucleus  contains 
the  somatic  number  of  chromosomes,  which  actually  appears  in  subse- 
quent divisions.  It  is  therefore  clear  that  while  such  parthenogenetic 
eggs  fail  to  receive  a  sperm  nucleus  they  retain  or  receive  back  the 
equivalent  of  such  a  nucleus  in  the  form  of  the  second  polar  body 
nucleus,  which  is  not  lost  as  it  is  in  eggs  requiring  to  be  fertilized. 


160  GENERAL  EMBRYOLOGY 

The  second  unusual  modification  of  maturation  is  to  be  seen  in  the 
spermatogenesis  of  many  Arthropods,  chiefly  Insects.  These  species 
have  already  been  mentioned  as  showing  a  numerical  difference  between 
the  chromosome  groups  of  the  male  and  female  individuals,  the  female 
having,  in  different  species,  one  or  several  chromosomes  more  than  the 
male.  In  these  forms  the  first  maturation  division  is  typical  and  the 
two  secondary  spermatocytes  are  similar.  But  the  second  maturation 
division  is  asymmetrical  in  that  one  or  more  chromosomes  known  as  the 
accessory  or  idiochromosomes  fail  to  divide  and  are  therefore  distributed 
to  only  one-half  of  the  spermatids  and  spermatozoa.  Half  of  the  sperm 

cells  then  have  ~  chromosomes,  the  other  half  -  plus  one,  or  more  as 

2i  '— 

the  case  may  be  in  different  species.  These  striking  phenomena  and 
their  relation  to  the  question  of  sex  determination  are  described  more 
fully  in  Chapter  VII. 

In  conclusion  we  should  mention  briefly  the  place  of  the  maturation 
divisions  in  the  life  histories  of  different  organisms.  In  any  many-celled 
organism  the  life  cycle  as  a  whole  may  be  said  to  consist  of  two  phases, 
one  characterized  by  the  possession  of  the  diploid  chromosome  group, 
the  other  by  the  haploid  group.  Among  all  of  the  Metazoa,  and  many 
of  the  Protozoa,  there  are  invariably  only  two  cell  generations  with  the 
haploid  number,  and  further,  these  always  are  the  two  final  generations 
in  the  process  of  gametogenesis.  Here  they  seem  bound  up  with  the 
process  of  fertilization  and  are  to  be  understood  only  from  the  point  of 
view  of  what  is  involved  in  this  process.  Considering  these  forms  alone 
it  is  difficult  to  understand  how  it  should  have  come  about  that  numerical 
reduction  of  the  chromosomes  should  occur  in  advance  of  the  condition 
out  of  which  arose  the  necessity  for  reduction,  namely,  the  fusion  of  the 
germ  nuclei.  But  this  arrangement  is  by  no  means  invariable.  In 
Amceba  diploidea  (Hartmann  and  Nagler)  reduction  does  actually  occur 
after  conjugation.  And  in  some  of  the  lower  plants,  such  as  many  of 
the  green  Algae  (ChlorophyceaB) ,  the  relation  between  fertilization  and 
numerical  reduction  is  that  which  apparently  must  have  been  the  more 
primitive.  In  these  forms  the  gametic  nuclei  contain  the  same  number 
of  chromosomes  (s)  as  do  the  somatic  or  vegetative  cells;  these  fuse 
forming  a  zygote  with  double  this  number  (2s).  This  fusion  is  then 
followed  immediately  by  two  maturation  divisions,  the  first  of  which  is 
usually  heterotypic,  which  result  in  the  formation  of  four  cells,  each 
again  with  the  original  vegetative  number  (s).  Certain  or  all  of  these 
four  cells  then  produce  the  body  of  the  new  organism,  all  the  cells  of 
which,  including  the  germ  cells  when  these  form,  have  this  same  chro- 
mosome number  (s) .  That  is,  numerical  reduction  of  the  chromosomes 
follows  syngamy,  a  relation  which  seems  more  understandable  than  the 
more  common  precedence  of  reduction.  In  describing  cases  like  these 


MATURATION  161 

we  might  say  that  the  somatic  or  vegetative  cells  and  the  germ  cells  all 
have  the  haploid  chromosome  group.  In  fertilization  the  diploid  group 
is  formed,  but  is  then  retained  through  only  two  generations,  after  which 
the  haploid  condition  is  restored.  In  other  words  the  predominating 
stage  in  the  life  cycle  is  that  with  the  haploid  chromosome  group,  the 
diploid  group  occurring  only  in  the  divisions  following  fertilization; 
"haploid"  is  here  synonymous  with  "somatic." 

In  many  other  plants,  such  as  the  ferns  and  mosses  among  others,  the 
life  cycle  is  more  equally  divided  into  two  distinct  periods,  one  carried 
on  with  the  haploid  (in  the  usual  sense  of  the  word),  one  with  the  diploid 
chromosome  group.  The  cells  of  the  fern  while  it  is  in  the  typical 
"fern-plant"  stage  have  the  diploid  group,  but  during  the  formation 
of  spores  by  this  plant,  reduction  occurs,  the  reduced  number  appearing 
in  the  spore  mother  cell.  And  in  all  of  the  cells  of  the  prothallus, 
derived  from  the  spore,  the  haploid  number  remains;  no  further  reduc- 
tion occurs  when  the  prothallus  forms  gametes.  The  diploid  number  is 
only  restored  by  the  union  of  two  gametes  in  the  formation  of  the  new 
fern  plant,  throughout  the  existence  of  which  it  is  retained.  It  is  a 
matter  of  considerable  theoretical  interest  that  the  familiar  alternation 
between  the  sporophyte  and  gametophyte  generations,  between  fern 
plant  and  prothallus,  for  example,  should  be  accompanied  by  a  corre- 
sponding alternation  between  the  diploid  and  haploid  chromosome 
groups.  We  may  relate  this  to  the  condition  in  the  green  Algae  by  saying 
that  the  number  of  cell  generations  following  fertilization,  in  which  the 
diploid  chromosomes  are  retained,  is  greatly  increased  and  the  number 
with  the  haploid  group  correspondingly  diminished,  indeed  in  most 
cases  here,  the  diploid  stage  is  of  greater  duration  than  the  haploid.  In 
the  higher  plants  (Gymnosperms  and  Angiosperms)  it  is  agreed  that  the 
prothallus,  i.e.,  the  stage  with  the  haploid  chromosome  group,  is  repre- 
sented only  by  certain  vestiges — the  pollen  tube  and  embryo  sac,  and  it 
is  significant  that  here,  after  the  two  maturation  divisions  leading  to  the 
formation  of  the  germ  cells,  two  or  more  (but  never  more  than  a  few) 
additional  divisions  occur  giving  rise  to  these  vestiges;  the  haploid 
chromosome  number  is  found  in  all  of  these  divisions.  Here  then  the 
phase  with  the  reduced  number  of  chromosomes  is  still  more  limited — 
practically  to  the  extent  found  in  animals.  And  whereas  in  the  lower 
plants  the  diploid  stage  is  restricted  to  two  cell  generations,  in  the  higher 
plants  it  is  the  haploid  stage  which  comes  to  be  so  limited. 

Many  consider  the  gametophyte  generation,  i.e.,  the  prothallus,  or 
its  equivalent  in  other  forms,  as  the  primary  form  or  phase ;  consequently 
they  regard  the  number  of  chromosomes  in  the  cells  of  this  phase,  the 
haploid  number,  as  primitive  or  normal,  and  not  as  a  reduced  number. 
Correspondingly  the  diploid  number  would  result  from  a  doubling,  not 
from  a  restoring  of  the  normal.  The  development  of  this  point  of  view 


162  GENERAL  EMBRYOLOGY 

in  connection  with  the  conditions  in  the  higher  plants  (Strasburger)  has 
led  to  the  suggestion  (Whitman)  that  even  in  animals  the  number  of 
chromosomes  in  the  secondary  06-  and  spermatocytes  and  mature  germ 
cells,  i.e.,  the  haploid  number,  is  again  in  reality  the  normal,  that  this 
is  doubled  in  fertilization,  and  remains  doubled  throughout  the  somatic 
divisions,  only  to  be  again  reduced  to  normal  by  the  subsequent  matura- 
tion divisions.  Upon  this  hypothesis,  which  also  explains  the  present 
precedent  relation  of  maturation  to  fertilization,  the  two  cell  generations 
immediately  preceding  fertilization  are  all  that  remain  of  the  primary 
phase  of  the  animal  life  cycle. 

The  alternation  between  the  sporophyte  and  gametophyte  in  ferns 
and  mosses  is  truly  an  alternation  of  generations  and  we  may  thus  see 
an  alternation  of  generations  even  in  the  higher  plants  where  there  may 
be  a  total  of  only  four  divisions  with  the  haploid  number — the  normal 
according  to  some.  If  this  is  allowed,  it  is  possible  that  in  animals 
where  there  are  but  two  of  these  corresponding  cell  divisions,  we  might 
still  speak  of  an  alternation  of  generations  as  well ;  the  equivalent  of  the 
gametophyte  would  then  be  represented,  vestigially,  only  by  the  cells 
with  the  haploid  chromosome  group,  i.e.,  primary  and  secondary  06- 
and  spermatocytes  which  form  the  gametes  proper — and  the  equivalent 
of  the  sporophyte  generation  would  be  represented  by  all  the  remaining 
generations  of  cells  which  we  commonly  think  of  as  the  true  organism, 
and  which  forms  "asexually"  the  06-  and  spermatogonia — the  equiva- 
lents then  of  the  spore  mother  cells.  Of  course  in  animals  the  matter 
is  complicated  greatly  by  the  separation  of  the  two  sexes  as  two  sepa- 
rate individuals.  Such  a  comparison  as  this  must  remain,  at  least  for 
the  present,  as  an  interesting  speculation  merely,  for  none  of  the 
Metazoa  offers  any  variations  in  the  maturation  process  which  shed  any 
light  upon  the  comparison. 

REFERENCES  TO  LITERATURE 

AGAR,  W.  E.,  The  Spermatogenesis  of  Lepidosiren  paradoxa.     Q.  J.  M.  S. 

57.     1911. 

VAN  BENEDEN,  E.     (See  ref.  Ch.  II.) 
BOVERI,  T.,  Zellenstudien  I.     1887.     (See  ref.  Ch.  III.) 
BRAUER,  A.,  Zur  Kenntniss  der  Spermatogenese  von  Ascaris  megalo- 

cephala.     Arch.  mikr.  Anat.     42.     1893.     Zur  Kenntniss  der  Rei- 

fung  des  parthenogenetisch  sich  entwickelnden  Eies  von  Artemia 

salina.     Arch.  mikr.  Anat.     43.     1894. 
CALKINS,  G.  N.,  and  CULL,  S.  W.     (See  ref.  Ch.  II.) 
CARDIFF,  I.  D.,  A  Study  of  Synapsis  and  Reduction.     Bull.  Torrey 

Botan.  Club.     33.     1906. 
COE,  W.  R.,  The  Maturation  and  Fertilization  of  the  Egg  of  Cerebratulus. 

Zool.  Jahrb.     12.     1899. 


MATURATION  163 

DAVIS,  B.  M.,  Nuclear  Phenomena  of  Sexual  Reproduction  in  Algae. 

Amer.  Nat.     44.     1910. 
FARMER,  J.  B.,  and  MOORE,  J.  E.  S.,  On  the  Maiotic  Phase  (Reduction 

Divisions)  in  Animals  and  Plants.     Q.  J.  M.  S.     48.     1904. 
FICK,  R.,  Ueber  die  Vererbungssubstance.     Arch.  Anat.  u.  Physiol. 

(Anat.  Abth.).     1907. 
GATES,  R.  R.,  The  Mode  of  Chromosome  Reduction.     Botan.  Gaz.     61. 

1911. 
GREGOIRE,  V.,  Les  Cineses  de  maturation  dans  les  deux  regnes.     L'unite 

essentielle  du  processus  meiotique.     Cellule.     26.     1910. 
HARTMANN,  M.,  und  NAGLER,  K.,  Copulation  der  Amoeba   diploidea, 

n.  sp.,  etc.     Sitz.-Ber.  Ges.  Nat.  Freunde.     Berlin.     4.     1908. 
HERTWIG,  O.,  Beitrage  zur  Kenntniss  der  Bildung,  Befruchtung  und 

Teilung  des  Tierischen  Eies.     I.     Morph.  Jahrb.     1.     1875. 
JORDAN,  H.  E.,  The  Spermatogenesis  of  the  Opossum  (Didelphys  vir- 

giniana)  with  special  Reference  to  the  Accessory  Chromosome  and 

the  Chondriosomes.     Arch.  Zellf.     7.     1911. 
KORSCHELT  UND  HAIDER,  Lehrbuch,  etc.     (See  ref.  Ch.  III.) 
McCLUNG,  C.  E.,  The  Chromosome  Complex  of  Orthopteran  Spermato- 

cytes.     Biol.  Bull.    9.     1905. 
MONTGOMERY,   T.   H.,   JR.,  The   Heterotypic   Maturation   Mitosis   in 

Amphibia  and  its  General  Significance.     Biol.  Bull.    4.     1903. 

(See  also  ref.  Ch.  II,  III.) 
MORSE,  M.  W.,  The  Nuclear  Components  of  the  Sex  Cells  of  four  Species 

of  Cockroaches.     Arch.  Zellf.     3.     1909. 
PLATNER,  G.,  (See  ref.  Ch.  III.) 

SCHAFFNER,  J.  H.,  Synapsis  and  Synizesis.     Ohio  Naturalist.     7.     1907. 
SCHONFELD,  H.,  La  Spermatoge"nese  chez  le  taureau  et  chez  le  mam- 

miferes  en  general.     Arch.  Biol.     18.     1901. 
DE  SINETY,  R.,  Recherches  sur  la  Biologic  et  1'anatomie  des  Phasmes. 

Cellule.     19.     1901. 
STRASBURGER,  E.,  Ueber  periodische  Reduktion  der  Chromosomenzahle 

im   Entwicklungsgang  der  Organismen.     Biol.  Cent.     14.     1894. 

English  Translation  in  Annals  Botany.     8.     1895.     Ueber  Reduk- 

tionsteilung.     Sitz.-Ber.  Akad.  Wiss.     Berlin.     1904. 
WILSON,  E.  B.,  (See  ref.  Ch.  II.) 
WINIWARTER,  H.  v.,  Recherches  sur  Povogenese  et  organogenese  de 

Fovaire  des  Mammiferes.     (Lapin  et  Homme.)     Arch.  Biol.     17. 

1900.     (See  also  Anat.  Anz.     21.     1902.) 
WIXHVARTER,    H.   v.,   and   SAINMONT,   G.,   Nouvelles   recherches   sur 

Povogenese  et  I'organogen&se  de  Tovaire  des  Mammiferes.     Arch. 

Biol.     24.     1909. 


CHAPTER  V 
FERTILIZATION 

THE  complex  processes  of  the  formation,  differentiation,  and 
maturation  of  the  germ  cells,  described  in  the  two  preceding 
chapters,  are  to  be  regarded  as  preliminaries  to  the  process  of 
fertilization.  They  can  be  understood  only  as  preparatory 
steps  leading  to  the  final  meeting  of  an  ovum  and  a  spermato- 
zoon, and  their  fusion  into  a  single  cell.  The  cell  thus  formed 
is  a  "new"  organism,  which  immediately  commences  a  long 
series  of  reactions,  collectively  termed  development,  leading 
finally  to  the  establishment  of  a  form  resembling  that  of  the 
individuals  from  which  the  fusing  germ  cells  were  themselves 
derived. 

Among  animals,  with  the  probable  exception  of  a  few  of  the 
simplest,  an  almost  invariable  condition  of  the  continued  ex- 
istence of  any  specific  form  of  protoplasm  is  such  a  periodic 
mingling  of  the  living  substance  of  two  individuals  of  the  same 
species.  The  few  exceptions  among  the  Metazoa  are  found  in 
the  rare  self-fertilizing  hermaphroditic  creatures,  and  even 
here  the  mingled  plasms  may  be  said  to  have  had  somewhat 
separate  histories,  although  formed  within  the  body  of  a  single 
organism.  Animals  which  reproduce  parthenogenetically,  or 
by  such  methods  as  budding  or  fission,  sooner  or  later  in  their 
life  history  exhibit  these  processes  of  germ  cell  formation  and 
fusion. 

Among  the  plants,  on  the  other  hand,  while  the  union  of 
germ  cells  may  be  a  frequent,  and  in  some  cases  a  necessary 
preliminary  to  the  formation  of  a  new  organism,  yet  other 
tissues  and  living  masses  composed  of  several  kinds  of  tissue, 
taken  from  almost  any  part  of  the  organism,  may  give  rise  to  a 
new  individual.  Many  species  of  plants  are  thus  normally 
propagated  by  cuttings  of  leaf,  stem,  or  root,  or  by  runners, 

164 


FERTILIZATION  165 

buds,  etc.  This  is  particularly  true  of  the  Begonias,  where 
even  a  few  cells,  from  almost  any  part  of  the  growing  plant, 
may  be  removed  and,  under  proper  conditions,  be  made  to 
form  a  new  complete  organism  capable  of  producing  typical 
germ  cells. 

The  questions  why  fertilization  should  be  necessary,  and  how 
fertilization  actually  accomplishes  the  results  which  obviously 
follow  it,  are  not  easy  to  answer.  But  the  essential  facts 
regarding  the  process  of  syngamic  fusion,  and  the  visible 
results  of  it,  are  clear.  We  shall,  therefore,  confine  our  atten- 
tion first  to  these;  then,  having  described  the  phenomena  of 
fertilization  we  may  consider  briefly  some  of  the  more  theoret- 
ical aspects  of  the  process. 

The  word  " fertilization"  is  a  general,  inclusive  term,  used  to 
denote  all  of  the  various  phenomena  concerned  in  the  meeting 
and  fusion  of  the  germ  cells  (gametes)  or  germ  plasms,  and  even 
some  of  the  results  of  such  a  fusion.  The  simpler  fact  of  the 
mere  fusion  of  gametes  is  more  precisely  termed  syngamy.  In 
all  of  the  Metazoa  syngamy  may  be  defined  as  the  meeting  of 
two  completely  specialized,  unicellular  gametes,  an  ovum  and 
a  spermatozoon,  derived  in  most  cases  from  two  individuals, 
and  their  subsequent  fusion,  nucleus  with  nucleus,  and  cyto- 
plasm with  cytoplasm,  into  a  single,  uninucleate  cell,  the  zygote. 
This  definition  is  not  completely  applicable  to  the  unicellular 
organisms,  for  in  these  the  gametes  are  usually  not  completely 
specialized,  sometimes  indeed  not  especially  differentiated  at  all. 
Such  forms  may  offer  some  suggestions  as  to  the  history  and 
significance  of  the  fertilization  process,  and  we  shall  return  to 
consider  this  subject  later  in  this  chapter. 

We  have  seen  in  Chapter  III  some  of  the  methods  by  which 
it  is  ensured  that  eggs  and  sperm  shall  be  brought  into  the  same 
general  region  or  into  fairly  close  proximity,  but  it  remains  to 
be  seen  how  the  ovum  is  actually  encountered  by  the  sperm 
cell.  Taken  altogether,  the  processes  leading  to  this  result  often 
become  very  complicated  and  special,  and  in  most  species  the 
probability  is;  very  high  that  practically  every  normal  egg 
produced  will  be  fertilized.  The  gametes  are  completely 


166  GENERAL  EMBRYOLOGY 

specialized  cells,  and  if  they  cannot  conjugate  and  develop, 
they  soon  perish,  not  able  even  to  remain  alive  long,  except 
in  a  few  special  instances.  Spermatozoa  discharged  freely 
into  the  water,  as  in  external  fertilization,  are  usually  able  to 
remain  active  only  a  few  minutes  or  hours.  But  when  fertili- 
zation is  internal  and  the  spermatozoa  are  received  into  some 
reproductive  cavity  of  the  female,  or  into  some  storage  cavity, 
they  may  remain  alive  and  able  to  function  under  appropriate 
conditions  for  a  much  longer  period:  various  observers  give 
the  following  specific  instances:  dog  and  rabbit,  eight  days; 
man,  seven  to  twenty  days;  fowl,  twenty  days;  the  bats  and 
some  snakes,  from  autumn  to  the  following  spring;  Salamandra 
maculosa,  from  summer  to  the  following  spring;  snails,  three 
years;  bees,  four  to  five  years. 

Whatever  the  particular  circumstances  connected  with  and 
ensuring  the  meeting  of  sperm  and  ovum,  the  medium  in  which 
it  occurs  is  a  fluid.  In  this  the  sperm  cells  are  in  active,  though 
apparently  random,  movement,  due  to  rapid  vibration  of  the 
flagellum  or  tail.  In  many  instances  their  movement  follows  a 
spiral  path  (Buller,  Adolphi),  either  close  or  open,  such  as  is 
common  among  flagellated  unicellular  organisms.  In  a  few 
rare  instances  (some  Crustacea)  the  spermatozoa  are  amoeboid. 

Fertilization  becomes  more  likely  when  direction  is  given  to 
the  active  random  movements  of  the  sperm  cells.  In  some 
instances  where  fertilization  is  internal,  the  movements  of  the 
sperm  seem  to  be  directed  by  ciliary  currents  of  the  oviduct  or 
other  passage.  Spermatozoa  tend  to  swim  against  such  a 
current,  and  thus  to  ascend  toward  the  eggs  which  are  being 
carried  down  the  passage  toward  the  exterior  (Lott).  Accord- 
ing to  the  interesting  observations  of  Lott,  human  spermato- 
zoa swim  at  the  rate  of  27  mm.  (i.e.,  about  550  times  their 
own  length)  in  7.5  minutes.  At  the  corresponding  rate  of 
progression  a  man  5.8  feet  in  stature  would  walk  a  mile  in 
12.4  minutes. 

There  appear  to  be  two  chief  methods  by  which  spermatozoa 
are  finally  brought  to  the  surface  of  the  ovum.  In  some  few 
forms  the  egg  is  said  to  give  off  a  chemical  substance  to  which 


FERTILIZATION  167 

the  active  sperm  are  attracted;  when  the  random  movements 
of  the  sperm  bring  them  within  the  sphere  of  chemical  influence 
of  the  egg,  their  movements  immediately  become  directed 
toward  the  unfertilized  egg.  Among  some  of  the  lower  plants 
it  is  known  that  weak  solutions  of  malic  acid  and  its  compounds 
attract  spermatozoids;  in  others,  solutions  of  cane  sugar  act 
similarly  (Pfeffer).  It  is  at  present  doubtful,  however, 
whether  in  many  animal  eggs  the  control  is  also  of  a  chemical 
nature  (Buller).  In  some  forms  the  stimulus  is  certainly  not 
of  a  chemical  sort,  but  is  a  contact  stimulus.  The  sperm  of 
many  fishes,  for  example,  swim  at  random  until  they  touch 
some  solid  object,  egg  or  other  body,  and  from  this  they  are 
apparently  unable  to  escape.  According  to  the  observations 
of  Drago  the  collection  of  the  spermatozoa  about  an  ovum  is 
unusual,  and  when  it  occurs,  it  is  the  result  of  agglutination. 
It  should  be  said  that  in  most  cases  it  is  doubtful  whether  the 
movements  of  the  sperm  really  are  given  direction  toward  the 
ovum,  and  what  the  nature  of  the  stimulus  may  be,  when  such 
is  the  case.  As  we  have  seen,  the  contact  between  sperm  and 
egg  results  chiefly  from  the  large  numbers  of  sperm  produced, 
and  from  their  general  proximity  to  the  egg  resulting  from 
special  habits  of  spawning,  copulation,  etc. 

When  the  ovum  is  naked,  or  surrounded  by  only  a  thin 
vitelline  membrane,  the  sperm  apparently  may  enter  at  almost 
any  point  on  the  surface  of  the  egg.  This  is  true  of  many  forms 
among  the  Medusa?,  Turbellaria,  Nemertea,  Annelida,  Echino- 
dermata,  Gasteropoda,  Cephalochorda,  Amphibia,  and  Mam- 
malia. Entrance  is  usually  said  to  be  effected  by  the  active 
swimming  movements  of  the  spermatozoon,  which  force  the 
sharp  acrosome,  adapted  to  this  purpose,  through  the  limiting 
surface  of  the  ovum,  into  its  superficial  cytoplasm.  But  here 
again  extended  evidence  is  lacking,  and  in  many  forms  the  egg 
is  known  not  to  be  a  wholly  passive  recipient  of  the  sperm,  but 
to  take  a  considerable  share  in  accomplishing  its  entrance. 
Thus  in  the  sea-urchin,  when  a  sperm  head  approaches  the  egg 
closely,  the  superficial  cytoplasm,  at  the  point  nearest  the  sperm, 
is  elevated  into  a  small  cone  or  papilla  called  the  attraction 


168 


GENERAL  EMBRYOLOGY 


cone  (Wilson).  This  not  only  rises  to  meet  the  spermatozoon, 
but  seems  to  aid  in  drawing  it  into  the  egg  (Fig.  85).  In  some 
instances  (e.g.,  Julus)  this  attraction  cone  may  be  quite  high 
and  may  contain  a  part  of  the  chromatic  substance  of  the  egg 
nucleus.  According  to  the  observations  of  Lillie,  the  sper- 
matozoon of  Nereis  is  clearly  drawn  into  the  egg  through 
the  activity  of  the  latter,  the  sperm  itself  taking  no  active 
part  in  the  process  (Fig.  86). 


«— - 
a 


FIG.  85. — Entrance  of  the  spermatozoon  into  the  egg.  From  Wilson,  "  Cell," 
H,  after  Metschnikoff ;  /,  after  Fol.  A.  Spermatozoon  of  Toxopneustes,  X  2000; 
a,  the  apical  body;  n,  nucleus;  ra,  middle-piece;  /,  flagellum.  B.  Contact  with 
the  egg-periphery.  C,D.  Entrance  of  the  head,  formation  of  the  entrance-cone 
and  of  the  vitelline  membrane  (v),  leaving  the  tail  outside.  In  some  other 
Echinoderms,  the  tail  may  enter  the  ovum.  E,F.  Later  stages.  G.  Appearance 
of  the  sperm-aster  (s)  about  three  to  five  minutes  after  first  contact;  entrance- 
cone  breaking  up.  H.  Entrance  of  the  spermatozoon  into  a  preformed  depres- 
sion. /.  Approach  of  the  spermatozoon,  showing  the  attraction-cone. 

When  the  egg  is  surrounded  by  membranes  of  some  thickness 
or  density,  the  spermatozoa  are  usually  unable  to  penetrate 
them  and  the  only  path  of  entrance  is  then  through  the  micro- 
pyle,  the  existence  of  which  is  an  adaptation  for  this  event. 
There  is  apparently  no  agent  directing  the  sperm  toward  this 
perforation  in  the  membranes;  the  finding  of  it  is  a  matter  of 
chance.  The  chance  is  not  small,  however,  that  some  sperma- 
tozoon will  enter  the  micropyle,  for  ordinarily  the  fluids  around 
the  egg  are  filled  with  a  swarm  of  sperm  cells.  As  already 
noted  the  micropyle  is  commonly  at  the  animal  pole  of  the 
egg,  though  at  the  vegetal  pole  in  a  few  instances  (some 


FERTILIZATION 


169 


Molluscs).  Thus  the  region  which  is  to  receive  the  sperma- 
tozoon is  already  determined,  and  frequently  the  cytoplasm  is 
considerably  modified,  in  the  region  just  beneath  the  micropyle, 
into  a  special  substance  concerned  in  the  receipt  of  the  sperm; 
this  is  known  as  the  entrance  disc  (Fig.  86). 


FIG.  86. — Entrance  of  the  spermatozoon  in  the  fertilization  of  the  Annulate, 
Nereis  limbata.  After  Lillie.  A.  Spermatozoon.  B.  Perforatorium  has  pene- 
trated egg  membrane;  entrance  cone  well  developed.  Fifteen  minutes  after 
insemination.  C.  Thirty-seven  minutes  after  insemination.  D.  Entrance  cone 
sinking  in  and  drawing  the  head  of  the  spermatozoon  after  it.  Forty-eight  and 
one-half  minutes  after  insemination.  E.  Head  drawn  in  still  further.  Forty- 
eight  and  one-half  minutes  after  insemination.  F.  Entrance  completed.  First 
maturation  division  in  anaphase.  Fifty-four  minutes  after  insemination.  The 
middle  piece,  as  well  as  the  tail,  remains  outside,  c,  head  cap;  e,  entrance  cone; 
h,  head  of  spermatozoon  (nucleus);  ra,  middle  piece;  p,  perforatorium ;  v,  vitelline 
membrane;  I,  first  polar  division  figure. 

With  but  comparatively  few  exceptions,  only  one  sperm  cell 
normally  enters  a  single  egg  (monospermy) .  This  sperm  is  the 
first  one  to  reach  the  egg  or  micropyle,  and  there  are  various 
methods  of  excluding  additional  sperm,  and  thus  of  preventing 


170  GENERAL  EMBRYOLOGY 

abnormal  multiple  fertilization.  In  some  of  the  lower  plants, 
after  one  sperm  cell  has  entered,  the  egg  gives  off  immediately 
a  chemical  substance  which  actually  repels  the  other  sperm 
congregated  about  the  egg.  A  frequent  method  among  animals 
is  the  secretion  of  an  impenetrable  membrane,  or  a  layer  of 
jelly,  immediately  upon  the  entrance  of  one  spermatozoon. 
Or,  if  a  membrane  was  previously  present,  its  density  may  be 
suddenly  increased,  or  an  additional  membrane  formed 


'I 


sp  1  sp 

FIG.  87. — Polyspermy  in  the  egg  of  the  Elasmobranch,  Torpedo  ocellata. 
From  Ziegler,  after  Riickert.  Germ  disc  with  first  cleavage  spindle,  /,  and  acces- 
sory sperm  nuclei,  sp. 

(Amphioxus),  or  the  micropyle  may  be  closed  by  the  rapid 
swelling  of  the  egg  membranes.  Although  this  process  of 
membrane  formation  may  really  have  this  effect  of  excluding 
supernumerary  spermatozoa,  the  general  significance  of  the 
process  renders  it  doubtful  whether  this  is  to  be  regarded 
primarily  as  an  event  adapted  toward  this  end. 

Occasionally  two  or  more  spermatozoa  succeed  in  gaming 
entrance  into  the  ovum  (polyspermy) .  This  ordinarily  results 
in  an  abnormal  course  of  development,  which  does  not  proceed 
very  far  before  the  egg  ceases  to  develop  and  dies.  A  few 
forms,  however,  are  adapted  to  the  receipt  of  more  than  one 
sperm  and  polyspermy  occurs  normally  (physiological  poly- 
spermy). Such  eggs  (Fig.  87)  are  usually  yolk-filled,  for 


FERTILIZATION  171 

example  those  of  some  Insects,  Petromyzon,  Selachians,  Uro- 
deles,  Reptiles,  Birds,  and  perhaps  the  toad  and  some  Teleosts. 
Sometimes  a  few,  sometimes  many,  sperm  thus  enter  the 
ovum,  but  in  any  case  only  one  of  them  ever  takes  any  real 
part  in  the  actual  processes  of  fertilization.  The  others,  known 
as  accessory  spermatozoa,  may  either  remain  quite  inactive  and 
soon  degenerate,  or  they  may  give  rise  to  "vegetative"  nuclei, 
and  perish  after  a  brief  period  of  activity.  While  active  they 
seem  chiefly  to  be  concerned  in  the  preparation  of  the  yolk  for 
ready  absorption;  they  are  then  called  merocytes.  Rarely,  if 
ever,  do  the  nuclei  derived  from  accessory  spermatozoa  con- 
tribute directly  to  the  formation  of  any  part  of  the  embryo 
proper. 

Apparently  there  is  little  specific  adaptedness  in  the  behavior 
of  the  germ  cells  such  that  an  egg  and  a  sperm  of  the  same 
species  tend  to  unite  much  more  readily  than  do  those  of  differ- 
ent species.  With  some  eggs  any  spermotozoon  that  is  morpho- 
logically capable  of  gaining  entrance,  can  do  so,  apparently 
about  as  readily  as  the  specific  sperm.  The  limitations  here 
are  frequently  due  to  the  size  of  the  sperm  head  as  compared 
writh  the  micropyle,  or  to  the  necessity  for  special  perforating 
mechanisms  or  powerful  swimming  movements  in  order  to 
penetrate  the  egg  membranes,  or  the  performance  of  appro- 
priate reactions  upon  the  part  of  the  egg  itself.  As  a  rule  the 
eggs  and  sperm  of  a  single  species  unite,  because,  as  the  result 
of  the  breeding  or  spawning  habits,  only  the  sperm  and  ova 
of  a  single  species  are  associated  in  time  and  space  in  any  con- 
siderable numbers.  When  eggs  are  placed  in  a  mixture  of 
equal  quantities  of  two  or  more  kinds  of  sperm,  there  seems  to 
be  no  appreciable  selective  fertilization,  provided,  as  said  above, 
that  both  or  all  kinds  of  the  sperm  are  able  to  enter  the  egg 
at  all. 

The  ease  with  which  "foreign"  sperm  may  enter  an  egg  is 
affected  hi  many  instances  by  chemical  treatment  of  the  eggs 
and  sperm;  treatment  with  alkalies  or  with  specific  salts  often 
renders  penetration  of  the  sperm  readily  possible  in  cases 
where  normally  it  is  difficult  or  impossible  (Loeb,  Godlewski). 


172  GENERAL  EMBRYOLOGY 

And  once  within  the  ovum,  a  "foreign"  sperm  seems  to  act 
almost  as  efficiently  as  the  proper  sperm  in  inaugurating 
development.  ,After  the  entrance  of  a  foreign  sperm  the  two 
germ  nuclei  may  not,  usually  do  not,  fuse,  and  other  internal 
developmental  processes  may  not  be  entirely  normal,  but  the 
external  processes  of  cleavage  and  differentiation  may  proceed 
normally  for  some  time,  even  to  the  formation  of  a  free-swimming 
larva,  as  in  many  species  of  Echinoderm  eggs  fertilized  by  the 
sperm  of  other  species,  genera,  or  even  of  other  classes  of  Echino- 
derms  (Baltzer),  or  of  other  phyla  (Mollusca,  Kupelwieser). 

In  many  species  the  entire  spermatozoon  enters  the  cyto- 
plasm of  the  ovum  (some  of  the  Turbellaria,  Annelids,  Insects, 
Molluscs,  and  many  Vertebrates)  while  in  others  the  tail  piece 
separates  from  the  remainder  of  the  sperm  cell  and  is  left 
outside  of,  or  embedded  within,  the  vitelline  membrane,  so 
that  only  the  head  and  middle  piece  actually  share  in  the 
formation  of  the  zygote.  In  some  instances  the  middle  piece, 
too,  fails  to  enter  the  egg  (e.g.,  Nereis,  Fig.  86).  Once  within 
the  egg,  the  sperm  continues  its  inward  course  for  a  short 
distance  only.  If  the  entire  sperm  cell  has  entered,  one  of  the 
first  events  is  a  sharp  bending  or  flexure  between  the  tail  and 
middle  piece,  often  followed  by  a  separation  of  the  two,  after 
which  the  tail  piece  is  left  behind  as  the  remainder  continues 
its  migration  (Fig.  88). 

The  entrance  of  the  spermatozoon  within  the  egg  cytoplasm 
is  the  event  which  inaugurates  a  whole  series  of  fertilization 
processes  culminating  in  the  formation  of  a  typical  mitotic 
division  figure  within  the  zygote.  The  precise  character  of  the 
stimuli  which  start  this  chain  of  actions  is  still  in  doubt,  but  it 
seems  likely  that  in  many  instances  it  acts,  first,  by  bringing 
about  the  formation  of  a  permeable  membrane  over  the  surface 
of  the  egg,  through  which  may  occur  rapid  and  extensive 
osmotic  interchanges  leading  to  marked  oxidations ;  and  second, 
by  reducing  the  amount  of  fluid  in  the  egg  cytoplasm,  either 
actually,  by  its  loss  through  the  permeable  membrane,  or 
relatively  by  the  addition  of  the  much  denser  substance  of  the 
spermatozoon  itself  (Loeb).  At  any  rate,  whether  it  be  a 


FERTILIZATION 


173 


FIG.  88. — Fertilization  in  the  sea-urchin,  Toxopneustes.  From  Wilson,  "Cell." 
A-F,  X  1067;(r,  X  533;  H,  I,  X  667.  A.  Sperm-head  before  entrance;  n,  nucleus; 
m,  middle-piece  and  part  of  the  flagellum.  B,C.  Immediately  after  entrance, 
showing  entrance-cone.  D.  Rotation  of  the  sperm-head,  formation  of  the 
sperm-aster  about  the  middle-piece.  E.  Casting  off  of  middle-piece;  centro- 
some  at  focus  of  rays.  F,G.  Approach  of  the  pronuclei;  growth  of  the  aster. 
H.  Union  of  pronuclei.  /.  Flattening  of  the  sperm  pronucleus  against  the  egg 
pronucleus;  division  of  the  aster. 


174 


GENERAL  EMBRYOLOGY 


primary  or  a  secondary  process,  this  loss  of  water  following 
the  entrance  of  the  spermatozoon,  appears  as  one  of  the 
important  aspects  of  fertilization. 

In  the  eggs  of  many  species  there  is  a  peripheral  layer  of 


FIG.  89. — Changes  in  the  structure  of  the  ovum  in  Nereis,  upon  fertilization. 
After  Lillie.  A.  Unfertilized  oocyte.  Large  germinal  vesicle;  cytoplasm  con- 
tains oil  drops  and  yolk  spheres,  and  shows  well  marked  cortical  layer  (exoplasm). 
B.  Fifteen  minutes  after  ensemination  (the  spermatozoon  is  not  shown).  The 
cortical  layer  has  chiefly  gone  to  form  a  jelly-like  layer  outside  the  ovum,  and  is 
not  shown.  C.  Egg  after  centrifuging  to  show  component  substances,  c, 
cortical  layer  (exoplasm);  n,  germinal  vesicle  or  nucleus;  no,  nucleolus;  o,  oil 
drops;  p,  perivitelline  space;  v,  vitelline  membrane;  y,  yolk  spheres;  1,  layer  of 
oil  drops;  2,  hyaline  cytoplasm  with  small  basophile  granules;  3,  yolk  spheres; 
4,  hyaline  cytoplasm  with  large  basophile  granules. 

cytoplasm  (exoplasm)  which  is  comparatively  clear,  free  from 
granules,  and  characterized  by  the  presence  of  fluid  vacuoles 
(Echinoderms,  Nereis  (Fig.  89),  Amphioxus  (Fig.  90),  Teleosts; 
see  Chapter  III).  Entrance  of  the  spermatozoon  leads  to 


FERTILIZATION 


175 


the  breaking  down  of  these  vacuoles  and  the  discharge  of  their 
substance  from  the  surface  of  the  ovum  (Figs.  89,  90).  This 
substance  may  be  in  part  transformed  into,  or  may  carry 
before  it  a  modified  surface  layer  of  material  which  appears  then 


FIG.  90. — Fertilization  in  the  egg  of  Amphioxus.  C,  after  Cerfontaine,  others 
after  Sobotta.  A.  Ovarian  egg  showing  cortical  plasm.  B.  Cortical  layer  form- 
ing a  membrane  on  the  surface  of  the  egg,  within  the  vitelline  membrane.  C. 
Egg  membrane  fully  formed  but  still  attached  to  surface  of  egg.  D.  Extruded, 
fertilized  egg.  Membrane  fully  formed  and  beginning  to  leave  the  surface  of 
the  egg.  c.  Cortical  layer;  e,  endoplasm;  m,  egg  membrane;  externally  vitelline, 
internally  a  product  of  the  exoplasm;  p,  perivitelline  space;  s,  spermatozoon; 
v,  vitelline  membrane;  /,  first  polar  body;  II,  second  polar  spindle. 

as  &  fertilization  membrane;  this  may  be  the  vitelline  membrane 
or  it  may  be  an  addition  to  a  previously  present  vitelline 
membrane  (Fig.  90).  Then  either  by  the  shrinkage  of  the  egg, 
or  the  expansion  of  the  membrane,  or  both,  or  by  the  rapid  ab- 


176 


GENERAL  EMBRYOLOGY 


sorption  of  water  by  the  substance  between  the  egg  and  the  mem- 
brane, a  space  of  widely  varying  dimensions  in  different  species, 
is  left  between  the  egg  and  its  membranes;  this  is  the  perivi- 
telline  space  (Fig.  90).  Within  this  space  the  egg  is  free  to 


FIG.  91. — Sections  through  the  egg  of  the  Tunicate,  Cynthia  partita.  After 
Conklin.  X  about  350.  A.  Ovarian  egg  fully  formed.  Germinal  vesicle 
surrounded  by  yolk  bodies;  peripheral  layer  of  protoplasm  containing  test  cells 
and  yellow  granules  (small  circles).  B.  After  extrusion  of  the  test  cells.  Nuclear 
membrane  still  intact  with  chromosomes  at  periphery  of  nucleus  (germinal 
vesicle).  C.  After  laying  (before  fertilization,  the  egg  remains  in  this  condition 
until  fertilized).  Chromosomes  and  granular  substance,  from  which  the  spindle 
is  formed,  lie  in  the  center  of  the  karyoplasm,  now  free  in  the  cell,  e,  exoplasm 
or  cortical  layer;  g,  granules  of  yellow  pigment;  n,  egg  nucleus  or  germinal  vesicle; 
t,  nuclei  of  ingested  test  cells  or  follicle  cells;  y,  yolk. 

move  or  rotate,  although  the  superficial  membrane  may    be 
fixed  to  some  foreign  body. 

Frequently  this  phenomenon  of  membrane  formation  is  but 


FERTILIZATION  177 

one  phase  of  a  general  physical  and  chemical  reorganization 
of  the  whole  substance  of  the  egg,  following  the  entrance  of  the 
spermatozoon.  The  egg  may  exhibit  more  or  less  amoeboid 
movement,  or  waves  of  contraction  may  pass  over  it.  A 
frequent  result  is  seen  in  the  rapid  streaming  of  differentiated 
cytoplasmic  substances  into  certain  regions,  where  these  specific 
substances  collect.  Thus  in  many  yolk-filled  eggs  like  those  of 
the  Teleosts,  the  protoplasm,  which  before  the  entrance  of  the 
sperm  is  quite  uniformly  distributed  over  the  surface  of  the 
egg  as  a  very  thin  layer,  now  collects  at  the  animal  pole  into 
a  thick  and  fairly  circumscribed  disc  called  the  germ  disc  (Fig. 
48).  The  Ascidian  egg,  as  described  by  Conklin,  offers  one  of 
the  most  marked  examples  of  this  rapid  transformation  and 
redistribution  of  the  substances  of  the  egg  cytoplasm  (Figs. 
91,  92).  In  the  secondary  oocyte  of  Cynthia  (Styela)  the 
greater  part  of  the  cell  is  composed  of  a  gray  "endoplasm"; 
superficially  there  is  a  thin  but  complete  layer  of  yellowish 
"mesoplasm";  while  the  large  nucleus  or  germinal  vesicle 
contains  a  clear  "  ectoplasm."  During  maturation,  which  here 
precedes  sperm  entrance,  the  ectoplasm  collects  at  the  upper 
pole  of  the  oocyte.  Immediately  upon  entrance  of  the  sperm 
the  yellow  mesoplasm  streams  from  all  directions  toward  the 
lower  pole;  this  is  followed  by  the  clear  ectoplasm  which  forms 
a  stratum  just  above  the  mesoplasm,  and  leaves  the  upper 
half  or  more  of  the  egg  cytoplasm  composed  entirely  of  the  gray 
endoplasm.  Then  this  radial  or  rotatory  symmetry  gives 
place  to  a  bilateral  symmetry,  for  the  mesoplasm  and  ecto- 
plasm move  up  on  one  side  (the  posterior)  of  the  egg,  appearing 
on  the  surface  in  the  form  of  a  crescent  just  below  the  equator. 
Meanwhile  the  yellow  mesoplasm  and  gray  endoplasm  have 
each  become  differentiated  into  two  distinct  substances,  so 
that  altogether  five  forms  of  protoplasm  are  distinguishable 
in  the  cytoplasm  of  the  zygote  (Fig.  92). 

Very  few  eggs  exhibit  such  marked  differentiation  as  this, 
but  the  corresponding  phenomena  are  of  widespread  occurrence, 
and  it  is  quite  likely  that  they  have  frequently  been  overlooked 
because  the  various  substances  are  not  often  marked  by 


178 


GENERAL  EMBRYOLOGY 


gv 


or 


E 


FIG.  92. — Total  views  of  the  egg  of  the  Tunicate  Cynthia  partita,  showing  the 
changes  in  the  arrangement  of  the  materials  of  the  egg  subsequent  to  fertiliza- 
tion. After  Conklin.  X  200.  A.  Unfertilized  egg,  before  the  fading  out  of  the 
germinal  vesicle.  Centrally  is  the  mass  of  gray  yolk;  peripherally  is  the  proto- 
plasmic layer  with  yellow  pigment,  and  surrounding  the  egg,  the  test  cells  and 
chorion.  B.  About  five  minutes  after  fertilization,  showing  the  streaming  of  the 
superficial  layer  of  protoplasm  toward  the  lower  pole,  where  the  spermatozoon 
enters,  and  the  consequent  exposure  of  the  gray  yolk  of  the  upper  hemisphere. 
The  test  cells  are  also  carried  toward  the  lower  pole.  C.  Side  view  of  egg  showing 
the  yellow  protoplasm  at  the  lower  pole;  at  the  upper  pole  a  small  clear  region 
where  the  polar  bodies  are  forming.  The  location  of  the  sperm  pronucleus  is  also 
indicated.  D.  Side  view  of  egg  shortly  before  the  first  cleavage,  showing  the 
posterior  collection  of  the  pigmented  protoplasm  (yellow  crescent)  and  the  clearer 
area  above  it.  E.  Posterior  view  of  egg  during  the  first  cleavage,  showing  its 


FERTILIZATION  179 

characters  so  easily  observed  in  the  living  egg.  It  is  also  note- 
worthy that  frequently  a  radial  or  rotatory  symmetry  of 
the  egg  is  changed  to  a  bilateral  symmetry  by  the  entrance 
of  the  spermatozoon,  and  that  usually  the  position  of  the 
plane  of  bilateral  symmetry  is  determined  by  the  point  at 
which  the  sperm  enters  or  by  the  path  which  the  sperm  takes 
through  the  cytoplasm.  And  further  this  new  plane  of  sym- 
metry of  the  zygote  coincides  closely  with  the  plane  of  the  first 
division  of  the  zygote  and  with  the  median  plane  of  the  embryo 
and  adult. 

These  aspects  of  organization  and  reorganization  of  the  egg 
are  among  the  highly  important  aspects  of  development,  and 
largely  determine  many  of  the  phenomena  of  subsequent 
differentiation.  They  also  illustrate  the  statement  made  in 
the  introductory  chapter,  that  some  of  the  most  important 
aspects  of  development  are  m/ra-cellular  processes.  We  shall 
return  to  this  subject  in  a  later  chapter  (Chapter  VII). 

After  the  entrance  of  the  spermatozoon  and  the  consequent 
redisposition  of  the  substances  of  the  cytoplasm,  the  course 
of  the  immediately  subsequent  events  is  determined  largely 
by  the  state  of  the  egg  nucleus  as  regards  its  maturation  process. 
For  in  most  cases  maturation  proceeds  only  to  a  certain  point, 
varying  greatly  in  different  forms,  when  it  pauses,  and  is  com- 
pleted only  after  receiving  the  stimulus  caused  by  the  entering 
sperm.  Ordinarily  the  state  of  the  egg  is  such  that  sperm  can 
gain  admission  only  when  this  pause  has  been  reached. 

In  some  cases,  such  as  Nereis  (Fig.  86),  Ascaris  (Fig.  94), 
Cerebratulus  (Fig.  95),  and  many  Molluscs,  it  is  true  that  the 
sperm  does  enter  the  ovum  before  the  maturation  process  has 
even  begun — while  the  egg  nucleus  (germinal  vesicle)  is  still 
in  the  final  resting  stage  preceding  the  first  oocyte  division. 
In  other  cases — most  Molluscs,  Thalassema,  Sagitta,  Teleosts, 
the  first  polar  spindle  has  formed  and  the  first  maturation 
division  may  have  reached  the  metaphase  or  anaphase,  when 

relation  to  the  symmetry  of  the  egg.  a,  anterior ;  c,  clear  protoplasm ;  cr,  yellow 
crescent;  e,  exoplasm  or  cortical  layer,  with  yellow  pigment;  g.v,  germinal  vesicle 
Ar,  chorion;  p,  posterior;  p.b,  polar  bodies;  t,  test  cells;  y,  yolk  (central  gray  mate- 
rial); y.h,  yellow  hemisphere;  cT,  sperm  pronucleus. 


180 


GENERAL  EMBRYOLOGY 


it  pauses  to  await  the  sperm  entrance.  In  Sycandra,  Lepas, 
Amphioxus  (Fig.  90),  and  many  Amphibia  and  Mammalia  the 
first  polar  division  is  completed  and  the  second  polar  spindle 
formed  before  the  pause.  And  finally  in  some  Ccelenterata 
and  most  Echinodermata  and  Ascidians,  the  maturation  of 
the  egg  is  entirely  completed  before  the  entrance  of  the  sperma- 
tozoon. In  such  cases  as  these,  contact  with  sea  water  seems 


FIG.  93. — Diagrams  of  the  two  most  frequent  relations  between  the  events  of 
maturation  and  fertilization.  From  Wilson,  "Cell."  I.  Polar  bodies  formed 
after  the  entrance  of  the  spermatozoon  (Annulates,  Molluscs,  Turbellaria) .  //. 
Polar  bodies  formed  before  the  entrance  of  the  spermatozoon  (Echinoderms).  A. 
Sperm  pronucleus  and  centrosome  at  d\  first  polar  body  forming  at  9  .  B. 
Polar  bodies  formed;  approach  of  the  pronuclei.  C.  Union  of  the  pronuclei. 
D.  Approach  of  the  pronuclei.  E.  Union  of  pronuclei.  F.  Cleavage  nucleus. 

to  furnish  the  stimulus  to  complete  the  maturation  process, 
which  may  be  begun  before  the  eggs  are  produced. 

We  may  distinguish  two  general  types  of  behavior  on  the 
part  of  the  germ  nuclei  according  to  whether  maturation  has  or 
has  not  been  completed  at  the  entrance  of  the  sperm  (Fig.  93). 
We  shall  consider  first,  and  at  greater  length,  those  cases  in 
which  maturation  is  not  yet  completed,  for  this  would  seem 
the  more  usual  course  of  events.  Otherwise  maturation  occurs 
precociously,  apparently  before  the  necessity  for  it  has  arisen. 


FERTILIZATION  181 

We  left  the  sperm  head  and  middle  piece  lying  a  short  distance 
below  the  surface  of  the  egg.  We  may  disregard  the  tail  piece 
now,  for  even  in  those  cases  in  which  it  enters  the  egg  it  is  left 
behind  the  head  and  takes  no  active  part  in  subsequent  proc- 
esses. The  head  and  middle  piece  now  move  more  slowly, 
along  a  path  which  is,  for  a  short  distance  at  any  rate,  a 
radius  of  the  ovum.  Then  they  separate  slightly,  and  the  two 
rotate  through  approximately  180°,  so  that  the  middle  piece  is 
placed  in  advance  of  the  head  (Figs.  94,  95).  There  ensues  a 
considerable  metamorphosis  of  these  elements.  The  sperm 
head  loses  its  sharp  outline  and  gradually  enlarges;  its  outline 
soon  becomes  very  irregular  and  indistinct,  and  vacuoles 
appear.  Soon  it  has  expanded  into  an  organ  of  considerable 
size  and  has  acquired  a  typical  nuclear  structure  with  linin 
network,  chromatin  granules,  and  nuclear  membrane.  In  the 
meantime  the  middle  piece  has  undergone  an  even  more  exten- 
sive transformation  (Fig.  94).  Before  the  sperm  halts  in  its 
inward  progress,  even  before  the  rotation  in  some  cases,  the 
middle  piece  has  begun  to  dissolve  and  in  connection  with  it 
appears  a  centrosome,  surrounding  which  a  small  aster  appears. 
During  the  pause  of  the  sperm  nucleus,  the  centrosome  and 
aster  each  divide  into  two,  and  the  daughters  diverge  slightly 
while  the  asters  grow  somewhat  larger. 

It  will  be  remembered  that  during  the  metamorphosis  of  the 
spermatid  into  the  spermatozoon,  one  or  both  of  the  centrosomes 
of  the  former  either  passed  into  the  middle  piece,  or  actually 
formed  the  larger  part  of  it,  although  frequently  no  centrosome 
is  actually  visible  in  the  middle  piece  of  the  fully  formed 
spermatozoon.  WTien  the  centrosome  appears  in  the  egg,  in 
connection  with  the  middle  piece  of  the  entering  spermatozoon, 
it  is  possible,  though  not  likely,  that  this  is  really  the  same 
centrosome  that  was  present  in  the  spermatid,  and  that  there 
is  consequently  a  genetic  continuity  of  centrosomes,  from  gen- 
eration to  generation,  as  well  as  of  nuclear  components.  Such 
a  continuity  has,  however,  not  been  definitely  observed.  On 
the  other  hand,  it  may  be  that  the  middle  piece  forms,  either 
from  its  own  substance,  or  from  that  of  the  egg,  a  new  centro- 


182 


GENERAL  EMBRYOLOGY 


FIG.  94. — Fertilization  in  Ascaris  megalocephala  bivalens.  From  Wilson, 
"Cell,"  after  Boveri.  (Later  stages  are  shown  in  Fig.  36.)  A.  The  spermato- 
zoon has  entered  the  egg,  its  nucleus  is  shown  at  cT;  beside  it  lies  the  granular 
mass  of  "archoplasm"  (attraction-sphere);  above  are  the  closing  phases  in  the 
formation  of  the  second  polar  body  (two  chromosomes  in  each  nucleus).  B. 
Germ-nuclei  (9  ,  d"1)  in  the  reticular  stage;  the  attraction-sphere  (a)  contains  the 
dividing  centrosome.  C.  Chromosomes  forming  in  the  germ-nuclei;  the  centro- 
some  divided.  D.  Each  germ-nucleus  resolved  into  two  chromosomes;  attrac- 
tion-sphere (a)  double.  E.  Mitotic  figure  forming  for  the  first  cleavage;  the 
chromosomes  (c)  already  split.  F.  First  cleavage  in  progress,  showing  divergence 
of  the  daughter-chromosomes  toward  the  spindle-poles  (only  three  chromosomes 
shown) . 


FERTILIZATION  183 

some.  The  aster  doubtless  is  formed  out  of  the  egg  cytoplasm, 
by  the  influence  of  the  centrosome  or  centrosome-forming 
substance  of  the  spermatozoon;  and  it  is  quite  possible  that  the 
centrosome  itself  may  be  similarly  formed  from  the  cytoplasm 
through  the  action  of  some  chemical  substance  introduced  by 
the  sperm.  For  it  is  known  that  the  cytoplasm  of  the  egg 
does  possess  the  property  of  forming  asters  with  typical  centro- 
somes,  under  the  influence  of  appropriate  " artificial"  stimulus 
(Yatsu).  And  recently  Lillie  has  shown,  in  Nereis,  where  the 
middle  piece  does  not  enter  the  egg  at  all,  that  the  centrosome 
forms  in  association  with  the  sperm  nucleus,  even  when  only  a 
small  portion  of  this  is  allowed  to  enter  the  egg.  This  indicates 
that  the  centrosome,  as  well  as  the  aster,  results  from  the 
redisposition  of  substances  of  the  egg  cytoplasm  following  the 
entrance  of  the  spermatozoon.  A  conclusion  as  to  whether  or 
not  the  law  of  genetic  continuity  applies  to  the  centrosome  in 
fertilization  is  less  important  than  recognition  of  the  uniformity 
of  its  chemical  and  physical  actions,  in  either  case.  And 
although  the  centrosome  as  an  organized  body  may  disappear 
in  the  spermatozoon,  this  still  contains  kinoplasmic  substance 
of  an  equivalent  function.  Here,  as  elsewhere,  the  essential  con- 
tinuity may  be  chemical  rather  than  morphological,  but  for 
that  reason  it  is  not  to  be  regarded  as  any  less  actual  or 
important. 

While  the  spermatic  structures  have  been  thus  active,  the 
egg  nucleus  has  been  completing  its  maturation,  at  the  con- 
clusion of  which  the  egg  centrosomes  and  asters  have  disap- 
peared (Figs.  94,  95).  The  egg  nucleus  is  left  near  the  surface 
of  the  animal  pole,  either  near  the  sperm  nucleus  or  at  some 
distance  from  it.  There  are  now  present  in  the  ovum  all  of 
the  chief  elements  which  are  to  take  part  in  the  essentials  of 
fertilization  and  development.  These  are  (1)  the  egg  nucleus 

o 

with  its  ^  chromosomes,  either  distinct  or  formed  into  a 
characteristic  nuclear  reticulum,  and  with  or  without  a  nuclear 

o 

membrane;  (2)  the  sperm  nucleus,  also  known  to  contain  ^ 


184 


GENERAL  EMBRYOLOGY 


G 


FIG.  95. — Fertilization  in  the  Nemertean,  Cerebratulus.  After  Coe.  C,  D, 
X  375,  others  X  250.  A.  Primary  oocyte.  Part  of  the  chromatin  has  been 
condensed  into  chromosomes,  only  five  of  which  are  shown  (the  number  present 
is  sixteen).  The  remainder  of  the  chromatin  is  thrown  out  into  the  cytoplasm. 
The  centrosomes,  each  with  a  small  aster,  are  diverging,  and  the  nuclear  mem- 
brane is  commencing  to  disappear.  B.  First  polar  spindle  fully  formed  and  ro- 
tated into  radial  position.  Chromosomes  in  equatorial  plate.  C.  First  oocyte 
division;  anaphase.  D.  First  polar  body  nearly  separated.  E.  First  polar  body 
completely  cut  off;  second  polar  spindle  formed  and  rotating  into  radial  position. 
Spermatozoon  within  the  egg.  F.  Second  polar  body  completely  separated. 
Egg  pronucleus  forming,  surrounded  by  large  aster.  Sperm  pronucleus,  also 
with  a  large  aster,  enlarged  and  approaching  the  egg  pronucleus.  G.  Approach 
of  the  two  pronuclei.  Egg  aster  reduced,  sperm  aster  greatly  enlarged  and 
centrosome  divided.  H.  Fusion  of  pronuclei;  divergence  of  the  sperm 
centrosomes.  /.  First  cleavage  figure  in  early  anaphase.  Chromosomes  divided 
and  beginning  to  diverge;  centrospheres  enlarged,  c,  chromosomes;  o,  nucleolus, 
vacuolated  and  commencing  to  disappear;  s,  spermatozoon  just  within  the  egg; 
v,  germinal  vesicle;  vc,  contents  (extra-chromosomal)  of  germinal  vesicle;  /,  //, 
first  and  second  polar  bodies;  d\  sperm  pronucleus;  9  ,  egg  pronucleus. 


FERTILIZATION  185 

chromosomes;  (3)  the  centrosomes  and  asters 'derived  in  some 
way,  either  directly  or  indirectly,  from  the  spermatozoon.  In 
syngamy,  therefore,  the  ovum  supplies  the  great  bulk  of  the 
cytoplasmic  basis  of  the  zygote,  together  with  one-half  the 
nuclear  material,  while  the  spermatozoon  furnishes  the  other 
half  of  the  nuclear  substance,  and  produces  the  centrosomes, 
which  here  as  elsewhere  are  to  be  regarded  as  the  dynamic 
centers  for  division.  It  should  not  be  overlooked  that  a  small 
amount  of  cytoplasm,  including  certain  mitochondrial  structures, 
does  accompany  the  sperm,  particularly  when  the  tail  piece 
enters  the  egg;  it  is  by  no  means  impossible,  though  not  at  all 
demonstrated,  that  this  cytoplasm  from  the  sperm  may  con- 
tain substances  of  great  importance  in  later  development  and 
differentiation.  In  brief,  however,  it  is  true  that  in  this  union 
of  gametes  the  ovum  is  the  material  factor,  the  Spermatozoon 
the  dynamic,  and  each  contributes  equally  to  the  nuclear  or 
controlling  mechanism. 

But  these  structures  are  as  yet  distributed  in  different  parts 
of  the  cell.  The  association  of  the  scattered  elements  into  a 
typical  mitotic  figure  now  follows  and  constitutes  the  final 
step  in  fertilization  and  the  formation  of  the  new  organism. 
Maturation  completed  and  the  sperm  nucleus  dissolved,  the 
two  germ  nuclei  commence  to  approach  one  another,  the  sperm 
nucleus  following  the  centrosomes  and  asters.  The  paths  of 
their  approach  are  seldom  directly  toward  one  another,  as 
they  are  in  some  of  the  Nematodes,  but  are  more  or  less 
curved  (Fig.  96),  and  seem  in  a  way  determined  by  some  factors 
other  than  mere  mutual  attraction,  though  this  is  doubtless  the 
essential  factor  in  their  movement  (Wilson). 

The  entrance  path  of  the  spermatozoon  is  frequently  marked 
by  cytoplasmic  modifications,  often  of  a  very  pronounced 
character,  giving  evidence  of  intense  metabolic  (katabolic) 
activity;  thus  we  might  note  the  frequency  of  the  accompanying 
formation  of  pigment,  which  is  usually  regarded  as  a  by-product 
of  protoplasmic  decomposition. 

We  have  already  seen  that  the  path  of  the  sperm  nucleus 
may  be  an  important  factor,  either  in  determining  or  in  making 


186 


GENERAL  EMBRYOLOGY 


evident,  the  position  of  the  plane  of  symmetry  of  the  zygote, 
and  hence  of  the  embryo.  In  a  few  cases  the  nuclei  are  some- 
what amoeboid,  in  others  they  seem  to  be  carried  by  proto- 
plasmic currents,  and  in  still  others  they  seem  directed  by  the 


FIG.  96. — Diagrams  showing  the  paths  of  the  germ-nuclei  in  four  different  eggs 
of  the  sea-urchin  Toxopneustes.  From  camera  drawings  of  the  transparent 
living  eggs.  From  Wilson,  "Cell."  In  all  the  figures  the  original  position  of 
the  egg-nucleus  (reticulated)  is  shown  at  9  ;  the  point  at  which  the  spermatozoon 
enters  at  E  (entrance-cone).  Arrows  indicate  the  paths  traversed  by  the  nuclei. 
At  the  meeting-point  (M)  the  egg-nucleus  is  dotted.  The  cleavage-nucleus  in 
its  final  position  is  ruled  in  parallel  lines,  and  through  it  is  drawn  the  axis  of  the 
resulting  cleavage-figure.  The  axis  of  the  egg  is  indicated  by  an  arrow,  the 
point  of  which  is  turned  away  from  the  micromere-pole.  Plane  of  first  cleavage, 
passing  near  the  entrance-point,  shown  by  the  curved  dotted  line. 

location  of  the  asters.  These  have  grown  to  a  considerable 
size  now,  and  seem  mechanically  forced  into  that  position  where 
the  tensions  between  the  cytoplasm  and  the  more  or  less  rigid 
asters  reach  an  equilibrium.  Most  frequently  this  is  the  center 
of  the  cytoplasmic  mass,  so  that  ultimately  the  two  nuclei 


FERTILIZATION  187 

approach  and  meet  in  this  region.  As  the  nuclei  come  into 
contact  the  two  asters  diverge  in  such  a  way  as  to  lie  at  opposite 
ends  of  a  tangent  drawn  through  the  point  of  nuclear  contact 
(Figs.  94,  95).  The  nuclear  walls  then  dissolve;  a  spireme 

o 

forms  in  each  nucleus  and  segments,  in  each,  into  »  chromo- 
somes, and  these,  as  a  typical  spindle  forms  from  the  egg 
cytoplasm,  become  arranged  at  its  equator.  The  result  is  the 
formation  of  a  typical  mitotic  figure  with  s  chromosomes.  This 
is  the  first  cleavage  figure,  and  here,  for  the  first  time  in  the 
existence  of  the  new  organism,  substances  of  paternal  and 
maternal  origin  are  associated,  on  equal  terms,  in  a  common 
structure. 

Before  we  mention  any  of  the  further  details  in  the  history 
of  this  cleavage  figure,  we  must  return  to  consider  briefly  the 
course  of  events  in  the  fertilization  of  those  eggs  which  are 
already  fully  mature  when  the  sperm  cell  enters  (Echinoderms, 
Ascidians).  In  such  cases  (Figs.  88,  93)  the  chief  divergences 
from  the  account  just  given  result  from  the  absence  of  any  pause 
of  the  sperm  nucleus  and  middle  piece  after  their  entrance. 
The  egg  nucleus  is  ready  for  fusion,  and  immediately  upon  the 
entrance  of  the  sperm  the  two  nuclei  proceed  toward  one 
another  as  described  above.  The  sperm  nucleus  thus  does  not 
have  time  to  be  dissolved  to  any  considerable  extent,  so  that 
when  the  two  nuclei  meet  they  are  by  no  means  of  equal  size 
(Fig.  88),  for  the  egg  nucleus  nearly  always  returns  to  its  "rest- 
ing" state  after  its  maturation  is  completed.  Nevertheless  it  is 
known  from  their  history  that  the  two  nuclei  are  equivalent  in 
chromosomal  composition.  Frequently,  too,  the  centrosome 
does  not  divide  until  just  as  the  two  nuclei  meet,  or  even  after 
they  have  begun  to  fuse.  The  two  centrosomes  accompanied 
by  asters  then  move  to  opposite  poles  of  the  combined  nuclei 
and  there  establish  the  mitotic  figure.  The  sperm  nucleus  in 
these  cases  does  not  become  resolved  into  a  typical  nuclear 
condition  until  after  its  fusion  with  the  egg  nucleus.  It  often 
results  from  this  that  the  two  nuclear  substances  seem  to  mingle 
quite  completely  before  the  spindle  is  formed  and  it  is  not  so 


188  GENERAL  EMBRYOLOGY 

easy,  as  in  the  cases  previously  described,  to  distinguish  at 
this  time  between  the  elements  derived  from  the  egg  and  sperm 
nuclei.  When  this  duplex  nucleus  forms  its  spireme,  this 
segments  into  the  somatic  number  of  chromosomes  immedi- 
ately, and  the  mitotic  figure  for  the  first  cleavage  forms 
typically. 

In  the  formation  of  the  first  cleavage  figure  we  see  the  net 
result  of  all  the  complex  processes  of  the  formation  and  matura- 
tion of  the  germ  cells,  and  the  union  of  the  two  gametes.  In  a 
word,  what  has  been  accomplished  is  the  reestablishment 
of  a  single  typical  cell  with  specific  organismal  characteristics. 
But  this  cell  now  has  a  nucleus  derived  in  equal  parts  from  two 
separate  individuals  of  the  species.  Into  this  nucleus  the 
events  of  maturation  have  made  it  possible  that  there  should 
have  been  brought  a  complete  and  equivalent  series  of  chromo- 
somes from  each  parent,  for  the  haploid  group  is  composed  of 
one  of  each  pair  of  chromosomes  of  the  diploid  or  somatic 
series.  And  from  this  nucleus  are  derived  all  of  the  nuclei  of 
the  developing  organism;  hence  every  cell  of  the  adult  body  may, 
probably  does,  contain  substance  derived  from  both  its  parents. 

We  may  regard  the  organization  of  these  two  haploid  series 
into  a  single  nucleus  as  the  culmination  of  the  whole  process  of 
fertilization.  Or,  on  the  other  hand  as  previously  suggested, 
we  may  consider  the  final  step  in  fertilization  as  not  occurring 
until  these  pairs  of  chromosomes  actually  fuse  in  synapsis 
during  the  maturation  of  the  succeeding  generation  of  germ 
cells.  From  this  point  of  view  the  actual  union  of  maternal 
and  paternal  structures  and  substance  never  occurs  in  the 
somatic  cells,  for  in  these  synapsis  is  not  known.  There  is 
involved  in  this  process  of  fertilization  much  more  than  these 
simple  morphological  facts  express,  and  to  this  subject  we 
shall  return  presently. 

We  shall  find  it  profitable  to  consider  now,  as  briefly  as  may  be,  the 
phenomena  of  fertilization  and  accompanying  gamete  formation  in  a 
series  of  unicellular  organisms  of  increasing  complexity  and  resemblance 
to  the  Metazoa.  This  subject  has  been  in  part  postponed  from  Chapters 
I  and  III,  and  it  should  be  stated  again  that  the  series  to  be  described 


FERTILIZATION  189 

• 

is  not  supposed  to  represent  a  phyletic  relation.  It  is  now  too  late  to 
state  with  any  considerable  degree  of  probability  the  course  of  evolution 
of  the  germ  cells  and  the  process  of  syngamy.  Apart  from  this,  how- 
ever, the  consideration  of  such  a  series  as  this  brings  out  many  important 
and  interesting  facts  regarding  the  general  process  of  fertilization,  and 
emphasizes  the  idea  that  this  complicated  process  as  we  see  it  in  the 
higher  organisms  to-day,  is  all  the  product  of  an  evolutionary  history. 

As  a  preliminary  distinction  of  a  general  and  underlying  character  we 
should  note  that  among  the  unicellular  forms  the  cells  which  meet  or 
fuse  may  be  of  the  same  race  or  family,  that  is,  closely  related  by  descent 
from  a  comparatively  recent  common  ancestor;  or  they  may  be  only 
distantly  related,  so  distantly  as  to  be  regarded  as  unrelated,  coming 
from  races  or  families  that  have  long  been  distinct,  and  that  have  had 
different  histories.  The  former  condition  is  termed  endogamy,  the  latter 
exogamy.  These  two  relations  may  be  distinguished  in  all  forms  of 
syngamy  or  conjugation;  exogamy  is  much  the  more  usual,  and 
involves  the  more  complicated  reproductive  processes  among  the 
Protozoa,  but  no  such  relation  seenjs  really  necessary,  for  conjugation 
may  occur  with  equal  facility  between  any  two  different  individual 
protoplasms,  whether  closely  related  or  not. 

In  a  general  way  we  may  arrange  the  varied  phenomena  of  conjuga- 
tion and  syngamy  in  three  classes  with  reference  to  the  nature  and 
extent  of  the  fusion  which  occurs.  In  its  simplest  form  this  fusion  is 
not  morphological,  but  is  expressed  by  the  congregation  of  cells  in 
groups;  this  is  to  be  regarded  as  a  form  of  cytotropy.  Occasionally 
large  collections  of  cells  result  and  the  elements  come  into  close  and 
extensive  contact  without  really  fusing  or  losing  cell  limits.  Whatever 
exchange  of  substance  there  may  be  occurs  through  an  osmotic  process. 
After  a  temporary  association  of  this  kind  the  cells  scatter  and  resume 
vegetative  and  reproductive  processes.  Such  a  process  of  cytotropy 
has  been  observed  in  Amoeba  (Rhumbler). 

The  simplest  form  in  which  a  real  fusion  of  plasmas  occurs  is  that 
known  as  plastogamy.  Here  two  or  sometimes  more  (2-30  in  Actino- 
phrys)  vegetative  cells  meet  and  flow  together  so  that  the  cytoplasms 
mingle  completely;  the  cell  nuclei  remain  separate,  though  osmotically 
they  may  affect  one  another  and  the  fused  cytoplasms.  The  result  of 
this  is  the  formation  of  a  physiologically  bi-  or  multinucleate  cell. 
Plastogamy  may  be  only  temporary;  in  such  a  case  the  cells  come  into 
relation  only  through  comparatively  limited  contact  surfaces  and  the 
original  cell  outlines  are  not  lost.  Then  after  a  brief  period,  during 
which  chemical  interchanges  may  occur,  the  cells  separate  again. 
More  frequently,  however,  plastogamy  is  permanent,  and  the  fusion  of 
the  cells  is  so  complete  that  the  original  cell  outlines  are  completely  lost. 
Then,  following  plastogamy,  the  nuclei  of  the  combined  cells  usually 


190 


GENERAL  EMBRYOLOGY 


divide  several  times  forming  a  considerable  number  of  smaller  nuclei; 
finally  the  cytoplasm  divides  correspondingly,  producing  thus  a  group  of 
zoospores  (brood  formation).  Such  processes  as  these  occur  most 
typically  in  the  Mycetozoa  (Myxomycetes)  and  also  in  such  forms  as 
Arcella,  Actinophrys,  and  some  Foraminifera  (Fig.  97). 

Finally  we  come  to  a  third  general  form  of  conjugation  known  as 
karyogamy,  which  as  the  word  indicates  involves  primarily  a  process  of 
nuclear  fusion  of  the  conjugating  cells,  although  accompanied,  of  course, 
by  cytoplasmic  fusion  which  may  be  of  hardly  secondary  importance. 
For  instance,  in  some  of  the  Infusoria  the  achromatic  spindles  fuse,  as 
well  as  the  nuclei  and  undifferentiated  parts  of  the  cytoplasm;  in  some 


FIG.  97. — Plastogamy  in  the  Rhizopod,  Arcella  vulgaris.  After  Elpatiewsky. 
A.  Plastogamic  union  of  about  five  individuals,  apparently  preparatory  to  the 
formation  of  zoospores  ("pseudopodiospores").  B.  Reproduction  (formation 
of  " macroamosbae ")  following  plastogamy.  c,  chromidia;  n,  nuclei;  d,  degen- 
erating nuclei, 

of  the  lower  plants,  even  the  plastids  of  the  gametes,  perhaps  also  their 
centrosomes,  fuse  together  during  conjugation. 

Most  of  the  more  familiar  fertilization  processes  of  the  Protozoa  are 
essentially  karyogamic,  but,  as  a  rule  (as  in  the  Metazoa),  not  all  of  the 
nuclear  substance  of  the  cell  is  involved  in  the  process.  For  usually, 
as  a  preliminary  to  conjugation,  the  vegetative  nucleus  gives  off,  into  the 
cytoplasm,  portions  of  its  substance  (chromidia) .  These  may  be  formed 
as  a  result  of  general  nuclear  disintegration,  or  the  nucleus  may  remain 
quite  intact  and  extrude  chromidia,  either  directly  through  its  mem- 
brane, or  by  a  process  of  nuclear  budding.  Some  of  these  chromidia 
are  concerned  in  reproduction ;  such  are  termed  idiochromidia.  Karyo- 
gamy, consequently,  involves  only  a  portion  of  the  nuclear  substance 
ordinarily,  and  the  remaining  chromidia  and  vegetative  nuclear  struc- 
tures may  even  break  down  and  disappear  during  the  process  of  ferti- 
lization. Altogether  these  processes  of  chromidia  formation  are  diverse 


FERTILIZATION 


191 


and  often  very  complicated  and  the  details  cannot  be  given  here. 
(Many  of  these  details,  and  references  to  the  literature  of  the  subject, 
are  given  by  Calkins,  " Protozoology,"  New  York,  1909). 

As  a  preliminary  to  the  description  of  typical  karyogamic  union  we 
may  refer  to  the  very  special  form  known  as  autogamy,  which  occurs 
in  many  of  the  simplest  Protista  (e.g.,  Entamceba,  Amoeba,  some 
Myxosporidia) .  In  autogamy  there  is  really  no  fusion  of  cells  at  all; 
the  characteristic  event  is  the  separation  of  the  nuclear  chromatic 


FIG.  98. — Autogamy  in  the  Flagellate,  Trichomastix  lacertce.  After  Prowazek. 
A.  First  nuclear  division  in  the  encysted  form.  B.  The  two  nuclei  completely 
separated.  C.  First  "reducing"  division.  D.  Second  "reducing"  division. 
E.  Approach  of  the  "reduced"  nuclei.  F.  Fusion  of  the  nuclei  to  form  a  single 
nucleus  (synkaryon). 

substance  of  a  single  cell  into  a  number  of  separate  bodies  (Figs.  98,  99), 
which  become  scattered  through  the  cytoplasm  as  chromidia,  or  rather 
as  idiochromidia,  for  they  are  concerned  in  reproduction.  After  their 
formation  is  completed  these  idiochromidia  fuse,  by  twos,  or  sometimes 
in  larger  groups,  forming  in  effect  "new"  nuclei,  or  at  any  rate  new 
combinations  of  chromatic  substance.  These  fused  chromatin  masses 
then  commonly  move  to  the  surface  of  the  cell  and  are  budded  off 
with  small  bits  of  cytoplasm,  as  small  cells  or  spores  (Fig.  99)  (Schau- 
dinn,  Calkins) .  Here  then  the  nuclei  which  fuse  together  are  the  direct 
derivatives  of  a  single  nucleus,  and  they  remain  within  the  same  cyto- 
plasmic  mass  throughout  their  formation  and  fusion.  This  might 
readily  be  regarded  as  an  extreme  form  of  endogamy;  it  suggests  the 
roughly  analogous  process  of  the  reentrance  of  the  nucleus  of  one  of 
the  polar  bodies  which  occurs  in  a  few  of  the  parthenogenetic  Arthropods. 
Fertilization  by  autogamy  is  considered  by  some  as  a  primitive  method 
of  fertilization  preceding  all  processes  of  gamete  formation  or  cell  fusions ; 
others  regard  it  as  a  derived  condition  in  which  the  nuclei  act  preco- 


192 


GENERAL  EMBRYOLOGY 


ciously.  However  this  may  be  it  seems  more  instructive  to  classify  the 
process  as  karyogamic. 

Coming  now  to  the  consideration  of  typical  karyogamic  fusion  we 
find  that  all  of  the  fertilization  processes  common  to  the  Metazoa,  as 
well  as  those  of  most  of  the  Protozoa,  belong  here.  And  here  again 
fertilization  may  be  either  endogamous  or  exogamous. 

Two  general  forms  of  karyogamy  proper  are  usually  distinguished, 
although  they  arc  so  clearly  connected  by  transitional  conditions  that 
they  must  be  regarded  as  merely  convenient  groupings.  These  are 
isogamy,  where  the  pairing  cells  are  similar  in  size,  form  and  behavior; 
and  anisogamy,  where  the  pairing  cells  are  markedly  dissimilar  in  size, 


FIG.  99. — Autogamy  in  the  Rhizopod,  Entamceba  histolytica.  From  Calkins, 
"Protozoology,"  after  Craig.  A.  Organism  showing  rods  and  granules  of 
chromatin  in  the  nucleus,  vacuole  with  some  stained  substance,  and  dense  ecto- 
plasm. B.  Chromatin  of  the  nucleus  passing  into  the  cytoplasm,  as  chromidia, 
shown  in  C.  D.  Aggregation  of  chromidia  to  form  secondary  nuclei.  E. 
"Spore  formation"  by  budding.  F.  Spores  formed  from  buds. 

form,  and  behavior.  That  is,  this  distinction  is  not  based  upon  differ- 
ences in  nuclear  structure,  or  behavior  during  union,  indeed,  these  are 
essentially  the  same  throughout  karyogamy,  but  upon  external  characters 
of  the  conjugating  cells. 

Considering  first  isogamy,  as  the  simpler  and  less  modified  process, 
we  find  it  restricted  to  the  unicellular  forms.  Isogamic  union  frequently 
occurs  between  two  individuals  of  the  usual  vegetative  type  which  do 
not  show,  externally  at  least,  any  structural  modifications  usually 
associated  with  gametic  behavior  (Fig.  100).  This  is  most  common 
among  the  Flagellates,  such  as  the  familiar  Copromonas,  and  Noctiluca, 
but  it  occurs  also  in  Aclinophrys  and  in  some  species  of  Amoeba.  In  other 
cases  the  conjugating  individuals  show  some  modification  in  form  as 


FERTILIZATION 


193 


compared  with  vegetative  individuals  (they  are  modified  similarly  of 
course),  but  there  is  no  reduction  in  size.  Thus  in  Cercomonas  and 
Tetramitus,  the  flagellate  bodies  of  the  conjugants  become  more  or  less 
amoeboid  and  distinctly  plastic  and  viscous.  Other  genera  exhibit 
various  stages  of  reduction  in  size  (Fig.  101),  although  in  form  they  still 
resemble  vegetative  cells.  This  is  the  case  in  most  of  the  Foraminifera 


FIG.  100. — Conjugation  in  the  Flagellate,  Copromonas  subtilis.  From  Calkins, 
"Protozoology,"  after  Dobell.  A.  Vegetative  form.  B.  Beginning  of  conjuga- 
tion of  two  organisms;  one  flagellum  withdrawn.  C.  Continued  fusion.  First 
stage  in  nuclear  "reduction."  D.  Second  "reducing"  division  (heteropolar). 
E.  Conjugation  completed;  "reduced"  nuclei  fusing.  F.  Zygote  within  cyst. 

and  many  other  Rhizopoda ;  it  is  rare  among  Flagellates  (Stephanosphcera, 
Chlamydomonas)  and  Ciliates.  Finally,  we  find  this  reduction  in  size 
accompanied  by  a  modification  in  form,  as  in  other  Rhizopoda  whose 
gametes  become  flagellated,  or  where  they  are  amoeboid  in  forms  usually 
flagellate  or  motionless. 

We  have  then  among  isogamous  organisms  a  series  of  forms,  at  one 
extreme  of  which  the  gametes  are  morphologically  unmodified,  at  the 
other  they  are  diminutive  and  structurally  modified,  usually  in  connec- 
tion with  the  motor  apparatus,  in  such  a  way  as  to  render  more  likely 
the  accident  of  their  meeting,  likelihood  of  which  is  largely  increased 


194 


GENERAL  EMBRYOLOGY 


through  the  fact  that  reduction  in  size  is  usually  the  result  of  multiple 
fission  or  brood  formation,  which  increases  the  number  as  well  as  the 
activity  of  the  gametes.  In  all  of  these  forms  of  isogamy  the  union 
of  the  gametes  is  permanent,  the  conjugants  fusing  completely  and 
thereby  losing  their  identity  as  individuals.  It  need  hardly  be  added 
that  in  these  cell  conjugations  the  essential  step  seems  to  be  the  fusion 
of  the  gametic  nuclei  into  a  single  zygote  nucleus. 


FIG.  101. — Gamete  formation  and  fusion  (isogamy)  in  the  Flagellate,  Chlamy- 
domonas  steinii.  After  Goroschankin.  A.  Vegetative  form,  n,  nucleus. 
B.  Group  of  gametes  formed  by  multiple  fission.  C.  Single  gamete.  D,  E,  F. 
Stages  in  the  fusion  of  gametes  (isogametes).  G.  Zygote. 

A  special  form  of  isogamy  needs  particular  notice  on  account  of  its 
frequency  among  the  most  familiar  Protozoa — the  Ciliata.  This  is  a 
temporary  form  of  isogamy  which  involves,  not  the  fusion  of  two  gametes 
to  form  a  zygote,  but  the  mutual  fertilization  of  the  two  gametes  through 
the  exchange  of  nuclear  substance  and  perhaps  also  a  small  amount  of 
cytoplasm.  The  details  of  nuclear  behavior  in  the  conjugation  of  Para- 
moecium,  for  example,  are  probably  familiar  but  will  bear  brief  restate- 
ment here.  This  outline  refers  particularly  to  that  form  of  Paramcecium 
having  only  a  single  micronucleus ;  one  should  recall  that  in  these  forms 
which  have  both  micronucleus  and  macronucleus,  the  former  is,  or 
represents,  the  idiochromidia,  the  latter  the  vegetative  nuclear  struc- 
tures. Two  Paramoecia  of  normal  vegetative  size  and  external  form 
meet  side  by  side,  oral  surfaces  in  contact,  in  a  sort  of  plastogamic  union, 
The  further  course  of  events  is  exactly  similar  in  each  individual  of 


FERTILIZATION 


195 


the  pair  (Fig.  102).     In  each  cell  the  micrenucleus  divides  and  the 
daughter   micro-nuclei    immediately   divide    again   forming   four.     Of 


FIG.  102. — Nuclear  history  during  conjugation  in  Param&cium  putrinum. 
After  Doflein.  The  macronuclear  structures  are  omitted  in  all  except  A.  A. 
Formation  of  spindle  for  first  division  of  micronucleus.  B.  Telophase  of  first 
division.  C.  Second  division  of  the  micronucleus.  D.  Degeneration  of  three 
of  the  micronuclei;  the  fourth,  or  permanent  micronucleus,  preparing  for  another 
division.  E.  Division  of  the  permanent  micronucleus  into  the  stationary  and 
migratory  micronuclei.  The  spindle  is  greatly  elongated  and  has  a  characteristic 
mid-body.  F.  Grouping  of  micronuclei  and  exchange  of  migratory  micronuclei. 
G.  Fertilization  (mutual).  Fusion  of  migratory  with  stationary  micronuclei. 
H.  Formation  of  spindle  for  first  division  of  the  fusion  nucleus  in  each  conjugant. 
/.  Late  phase  of  second  division  of  fusion  nucleus.  J.  Third  division  of  fusion 
nucleus.  K.  Exconjugant  with  eight  micronuclei,  derived  from  the  fusion 
nucleus.  (The  degenerating  micronuclei  are  omitted  from  G-K.)  d,  degener- 
ating micronuclei;  m,  migratory  micronucleus;  ma,  macronucleus;  mi,  micro- 
nucleus;  p,  permanent  micronucleus;  s,  stationary  micronucleus;  sp,  spindle. 

these,  three  degenerate   (cf.  maturation),  while    the    one    remaining 
divides  once  more,  this  time  unequally,  forming  a  larger  and  a  smaller 


196  GENERAL  EMBRYOLOGY 

micronucleus  in  each  organism.  Each  larger  or  " stationary"  micro- 
nucleus  remains  passive,  but  the  smaller  or  "  migratory"  nucleus  becomes 
active  and  moves  through  the  bridge  of  fused  cytoplasms  to  the  sta- 
tionary micronucleus  of  the  other  individual,  with  which  it  fuses  forming 
a  single  compound  or  zygotic  nucleus  in  each  individual.  The  two 
Paramoecia  now  separate  each  with  a  nucleus  of  modified  composition. 
The  macronucleus,  which  has  taken  no  share  in  the  events  of  fertilization, 
now  fragments  and  dissolves  leaving  the  fusion  nucleus  as  the  only 
nuclear  structure  present.  Then  by  three  successive  divisions  the 
fusion  nucleus  gives  rise  to  eight  small  nuclei ;  four  of  these  in  the  pos- 
terior end  of  the  cell,  remain  small,  as  micronuclei,  while  the  other  four, 
in  the  anterior  end,  enlarge,  forming  macronuclei.  During  the  first 
fission  of  this  cell  each  daughter  cell  receives  two  nuclei  of  each  kind, 
and  at  the  next  division  each  of  the  four  granddaughters  of  the  "  zygote  " 
receives  one  micronucleus  and  one  macronucleus,  and  the  normal 
vegetative  condition  is  restored.  This  form  of  karyogamy  is  peculiar 
for  at  least  three  reasons;  the  nuclei  alone  fuse,  both  of  the  gametes 
undergo  nuclear  reconstruction,  and  the  individuality  of  the  gametes  is 
not  lost. 

The  sessile  Ciliates  show  an  adaptive  modification  of  this  process  which 
is  anisogamic  in  character.  In  the  common  Vorticella,  for  example, 
while  one  of  the  conjugants  or  gametes  retains  its  normal  vegetative 
form,  the  other  is  small  and  one  of  a  brood  of  four,  which  become  free- 
swimming.  A  small  individual  upon  meeting  a  large  one,  is  actually 
absorbed  by  it.  The  early  nuclear  history  of  each  organism  is  much  the 
same  as  in  Paramcceium,  save  that  the  final  micronucleus  of  the  mega- 
gamete  is  one  of  four,  that  of  the  microgamete  one  of  eight,  the  remainder 
in  each  gamete  having  degenerated.  But  after  the  equivalents  of  the 
stationary  and  migratory  micronuclei  (idiochromidia)  are  formed 
in  each  gamete,  the  process  changes  somewhat,  for  now  one  of  the  two 
micronuclei  (the  equivalents  of  one  stationary  and  one  migratory  body) 
degenerates  in  each  organism,  while  those  remaining  fuse  together 
forming  thus  only  a  single  fusion  nucleus  in  the  single  but  duplex  zygote. 
Thus  there  is  no  mutual  fertilization,  and  while  strictly  this  is  anisog- 
amous,  it  is  mentioned  here  because  it  is  clearly  derived  from  the  more 
typical  Ciliate  condition  as  an  adaptation  to  the  sessile  life  of  the  vege- 
tative form. 

Coming  now  to  anisogamous  karyogamy  (Fig.  103),  we  should  note 
that  transitional  conditions  between  isogamy  and  anisogamy  are  not 
infrequent.  Thus  in  the  Flagellate,  Bodo,  the  conjugants  may  be 
either  of  equal  or  unequal  size,  apparently  in  an  accidental  fashion. 
The  colonial  Pandorina  forms  gametes  of  three  sizes,  small,  medium, 
and  large,  and  conjugation  may  occur  between  any  smaller  and  any 
larger  individuals  anisogamically,  or  the  small  or  the  medium  organisms 


FERTILIZATION 


197 


may  conjugate  together  isogamically.  Here  then  anisogamy  is  not 
obligatory,  but  facultative  or  accidental.  In  this  case  exogamy  is  the 
rule  since  a  single  colony  forms  gametes  of  one  size  only,  though  in 
isogamy  endogamy  may  occur. 


FIG.  103. — Formation  of  gametes  and  syngamy  in  the  Sporozoan,  Klossia 
octopiana.  From  Calkins,  "Protozoa,"  after  Siedlecki.  The  chromatin  of  the 
nucleus  is  distributed  throughout  the  cell,  A,  B,  finally  forming  nuclei  of  the 
future  gametes,  C,  D,  E.  The  mature  gametes,  s,  swim  about,  and  join  a  macro- 
gamete,  F.  The  nuclei  mingle,  G,  and  then  the  cleavage  nucleus  divides  repeat- 
edly by  mitosis,  to  form  the  spores,  H.  d",  microgametic  nucleus. 

In  true  anisogamy  conjugation  is  practically  always  exogamous  for 
as  a  rule  a  single  organism  forms  gametes  of  only  one  size  at  a  time. 
The  essential  difference  between  the  gametes  is  probably  that  of  behav- 
ior, i.e.,  degree  of  activity,  associated  with  which  are  constant  differences 
in  size.  The  larger  gametes  or  megagametes,  are  less  numerous  and  less 


198 


GENERAL  EMBRYOLOGY 


active  than  the  smaller  microgametes,  which  may  be  formed  in  very 
considerable  numbers.  Occasionally  the  gametes  differ  in  form  only, 
as  in  Dallingeria,  where  one  of  the  gametes  has  three  flagella,  like  the 
vegetative  cells,  while  the  other  gamete  has  but  one  flagellum.  In  a 


E 


FIG.  104. — Micro-  and  macrogametes  of  various  Protozoa,  illustrating  various 
degrees  of  differentiation.  After  Doflein,  from  various  authors.  /,  F,  x  562, 
others  X  1125.  a,  A.  Urospora  lagidis  (Brasil).  6,  B.  Collozoum  inerme 
(Brandt),  c,  C.  Chlamydomonas  braunii  (Goroschankin).  d,  D.  Volvox  aureus 
(Klein),  e,  E.  Cyclosporia  caryolytica  (with  two  polar  bodies  in  cytoplasm  of  E) 
(Schaudinn).  /,  F.  Orcheobius  herpobdella  (Kunze). 

few  forms  such  as  Monas  and  many  of  the  Gregarines,  the  only  morpho- 
logical difference  between  the  gametes  is  that  of  size,  the  microgamete 
being  smaller  and  somewhat  the  more  active,  but  not  otherwise  unlike 


FERTILIZATION  199 

the  megagamete.  In  other  forms  the  microgametes  are  considerably 
modified  structurally,  usually  in  connection  with  an  increase  in  loco- 
motor  activity.  At  the  same  time  the  megagamete  may  increase  con- 
siderably beyond  the  ordinary  vegetative  size  and  may  then  lose  motility 
more  or  less  completely.  So  it  finally  comes  about  that  gametes  of  two 
wholly  different  types  are  formed,  both  quite  unlike  vegetative  cells, 
and  the  typical  Metazoan  condition  is  reached. 

Several  groups  of  Protozoa,  e.g.,  Gregarines,  colonial  Flagellates, 
afford  interesting  series  showing  stages  in  this  differentiation  of  the 
gametes  (Fig.  104).  We  may  outline  one  such  series  selecting  examples 
from  the  Volvocine  group  of  Flagellates. 

In  Stephanosphcera  all  the  individuals  of  the  colony  are,  or  may  be, 
reproductive,  and  conjugation  is  isogamous  and  endogamous.  There 
is  no  differentiation  of  gametes.  In  Pandorina  (Fig.  8)  all  of  the  indi- 
viduals may  be  reproductive,  but  some  of  the  gametes  may  be  differ- 
entiated in  size,  and  conjugation  may  be  either  isogamous  or  anisogamous 
as  described  above.  Here  dissimilarity  of  the  gametes  is  facultative. 
In  Eudorina  two  kinds  of  colonies  are  found.  In  one,  all  the  cells  may 
become  reproductive,  the  individuals  forming  megagametes  only  slightly 
larger  than  vegetative  cells ;  in  another  only  four  cells  of  the  colony  are 
reproductive  and  each  of  these  forms  sixty-four  very  small  and  active 
microgametes.  Fertilization  is  here  strictly  anisogamous  and  exo- 
gamous.  The  last  step  is  represented. by  Volvox  (Fig.  10),  where  the 
number  of  gamete  forming  cells  is  always  limited.  Here  too  differentia- 
tion of  the  gametes  reaches  its  climax  among  the  Protozoa,  and  the 
Metazoan  condition  is  reached.  The  reproductive  cells  lose  their  motor 
organs  and  begin  to  enlarge.  A  few  of  them  grow  to  a  relatively 
enormous  size  and  become  the  passive  megagametes.  The  others 
grow  to  lesser  extent  and  then  divide  rapidly,  each  forming,  probably 
128  microgametes.  These  are  very  small  flagellated,  extremely  active 
cells,  with  an  elongated  rostrum  or  penetrating  organ  at  one  end  (Fig. 
104,  d).  The  microgametes  are  liberated  in  large  numbers  and  swim 
about  until  one  reaches  a  megagamete  which  it  then  enters  and  their 
nuclei  fuse  forming  a  typical  zygote  which  then  reproduces  a  new 
colony.  The  resemblance  to  the  gametes  of  the  Metazoa  is  so  com- 
plete that  they  are  here  termed  the  oosphere,  or  ovum,  or  oogamete 
(megagamete)  and  the  spermatozoon  or  spermagamete  (microgamete) . 

It  should  perhaps  be  noted  here  that  the  process  of  conjugation  or 
fertilization  is  not  always  associated  with  the  reproduction  of  the 
Protozoa  mentioned  above,  not  even  in  the  colonial  forms.  For  the 
usual  reproductive  processes  are  carried  out  by  the  simple  fission  of 
ordinary  vegetative  cells.  In  the  simpler  colonial  forms,  such  as 
Pandorina  and  Eudorina,  as  many  new  colonies  may  be  formed  as  there 
are  individuals  forming  the  original  colony,  in  Volvox,  however,  the 


200  GENERAL  EMBRYOLOGY 

number  of  cells  which  may  reproduce  in  this  way  is  limited,  and  there 
seems  to  be  a  real  distinction  between  soma  and  germ,  much  like  that 
of  the  Metazoan  organism,  the  mother  cell  which  divides  to  form  the 
multiple  spermatozooids,  has  even  been  compared  with  the  testis  of  a 
Metazoan. 

The  principal  forms  of  fertilization  and  gamete  formation  are  sum- 
marized in  the  accompanying  table. 

Several  facts  of  prime  importance  are  to  be  drawn  from  this  account. 
(a)  Among  the  Protozoa,  as  well  as  the  Metazoa,  the  process  of  fertiliza- 
tion is  widespread.  (6)  Out  of  a  variety  of  forms  comes  that  form  of 
fertilization  characteristic  of  the  Metazoa,  namely,  karyogamy.  (c) 
Accompanying  this  karyogamy  is  a  gradual  and  finally  complete 
differentiation  of  gametes,  which  differ  morphologically  and  physio- 
logically, both  from  vegetative  cells  and  from  each  other,  (d}  The 
Metazoa  show  much  less  diversity  than  the  Protozoa  respecting  the 
process  of  fertilization  and  the  form  of  the  gametes. 

Such  a  series  of  stages  as  that  outlined  above,  of  the  gradual  differ- 
entiation and  specialization  of  gametes,  cannot  fail  to  suggest  the 
general  subject  of  sex.  It  does  indeed  indicate  the  nature  of  the 
original  distinction  between  the  sexes.  Among  the  Metazoa  the  pri- 
mary and  familiar  facts  upon  which  the  definition  of  sex  is  based,  are, 
that  spermatozoa-producing  individuals  are  males,  ova-producers  are 
females.  In  all  cases  of  isogamic  conjugation  no  distinction  between 
the  gametes,  and  therefore  between  sexes,  obtains.  In  those  instances 
transitional  between  isogamy  and  anisogamy,  we  may  see  the  begin- 
nings of  sex  distinction,  often  facultative.  True  anisogamy  involves  true 
sex  distinction;  at  first  relatively  slight  (Pandorina),  in  such  forms  as 
Volvox  or  Coccidium  and  many  other  Sporozoa,  the  fundamental 
differentiation  of  sex  seems  to  be  completely  established,  i.e.,  the 
gametes  are  markedly  unlike  and  conjugation  occurs  only  between  two 
dissimilar  cells. 

The  essential  processes  of  fertilization  are  entirely  equivalent  in 
isogamy  and  anisogamy,  so  that  the  fundamental  distinction  of  sex  is 
based  only  upon  the  external  form  and  behavior  of  the  gametes,  not 
upon  any  differences  in  the  nature  of  the  conjugation  processes  in  sexual 
and  non-sexual  forms,  for  none  exist. 

Most  of  the  colonial  Protozoa  are  monoecious  or  hermaphroditic, 
producing  gametes  of  both  kinds,  but  cross-fertilization  (exogamy)  is 
the  rule  here,  as  it  is  among  the  hermaphroditic  Metazoa,  and  frequently 
for  the  same  reason  in  both  groups,  namely,  a  difference  in  the  times  of 
ripening  of  the  two  forms  of  gametes  of  a  single  colony  or  individual. 
Many  of  the  Sporozoa  are  clearly  dioecious  or  unisexual,  and  in  some  of 
these  there  are  also  secondary  sexual  characters,  usually  size  differences, 
such  that  macrogamete-forming  or  female  individuals  can  be  distin- 


FERTILIZATION 


201 


I 


202  GENERAL  EMBRYOLOGY 

guished  from  microgamete-f orming  or  male  individuals,  some  time  before 
the  gametes  are  actually  formed;  in  a  few  rare  instances  this  distinction 
can  be  made  throughout  the  life  cycle,  and  individuals  can  be  identified 
at  any  time  as  males  or  females. 

We  now  come  to  a  consideration  of  the  meaning  and  theo- 
retical significance  of  these  processes  of  fertilization  or  syngamy. 
Probably  there  is,  in  the  whole  field  of  Biology,  no  process  of 
such  widespread  occurrence  and  obvious  importance,  where 
the  phenomena  are  so  well  known,  which  at  the  same  time  is  so 
little  understood.  Why  fertilization  should  occur,  what  is 
effected  by  it,  and  how  syngamy  brings  about  the  results  which 
do  follow  it,  are  questions  to  which  to-day,  after  decades  of 
speculation  and  research,  no  sure  answers  can  be  given. 

Although  we  may  be  on  uncertain  ground,  it  will  be  profitable 
to  review  some  of  the  suggestions  and  hypotheses  that  have 
been  proposed  in  this  connection,  even  if  we  accomplish  little 
more  than  to  point  out  the  possibilities  and  difficulties  of  this 
fascinating  subject.  And  furthermore,  while  no  thoroughly 
demonstrated  solutions  of  the  problems  of  fertilization  have 
been  reached,  there  are  several  carefully  worked  out  hypotheses 
in  the  field,  some  of  which  are  certainly  to  be  regarded  as  close 
approximations  toward  the  correct  explanation  of  some  of  the 
problems  of  fertilization.  We  may  conveniently  arrange  these 
current  ideas  as  to  the  results  and  primary  "  purpose"  of  fertili- 
zation in  four  groups.  The  results  of  fertilization  may  be 
connected  with  (a)  reproduction,  (&)  rejuvenation,  (c)  the 
process  of  variation,  (d)  the  process  of  heredity.  In  considering 
each  of  these  we  shall  state  as  briefly  as  possible  the  essentials  of 
the  evidence  for  and  against  the  central  idea. 

That  fertilization  is  primarily  a  reproductive  process  was  the 
original  view  .held  by  Harvey  and  his  successors.  There  are 
now  two  forms  of  the  hypothesis.  In  one  form  we  find  the 
idea  that  the  ovum  quite  obviously  seems  to  contain  only  a 
part  (i.e.,  the  cytoplasm  and  one-half  a  nucleus)  of  the  mechan- 
ism necessary  for  development,  and  that  the  spermatozoon 
brings  into  the  ovum  those  parts  (i.e.,  one-half  a  nucleus  and 
the  centrosome,  or  the  stimulus  to  its  formation)  which  com- 


FERTILIZATION  203 

plete  this  mechanism  and  enable  development  to  proceed.  In 
its  other  form  this  idea'  is  that  the  ovum  is  a  quiescent,  passive 
body  which  needs  to  be  stimulated  to  its  normal  activity 
(development)  by  the  entrance  of  the  spermatozoon,  the  kino- 
plasm  (centrosome)  of  which  is  the  part  chiefly  acting  as  the 
stimulus.  In  short,  fertilization  is  to  be  regarded  normally  as 
the  necessary  antecedent,  as  the  cause  of  development. 

There  are  many  facts  opposed  to  this  view.  Of  these  we  shall 
discuss  two  chief  classes,  first,  those  of  parthenogenesis,  both 
normal  and  "artificial,"  and  second,  those  drawn  from  the 
relation  between  fertilization  and  reproduction  among  the 
Protozoa. 

For  present  purposes  we  may  extend  the  definition  of  partheno- 
genesis to  include  all  those  cases  where  single  cells,  specialized 
for  the  purpose,  develop  without  undergoing  syngamy.  In 
the  plant  kingdom  parthenogenesis,  in  this  broad  sense,  is  very 
widespread.  Development  from  spores  is  very  common,  even 
among  the  higher  (vascular)  plants,  and  in  some  instances 
(some  of  the  Fungi)  reproduction  by  single  unfertilized  cells  is 
the  exclusive  method.  And  the  development  of  ova,  typical 
in  every  respect  save  that  of  needing  to  be  fertilized,  is  not 
uncommon.  Among  the  single-celled  animals  phenomena 
equivalent  to  development  from  spores  are  frequent,  and  among 
the  multicellular  animals  normal  parthenogenesis  is  known  in 
the  Rotifera,  some  of  the  Crustacea,  and  in  several  orders  of 
Insecta.  In  most  of  these  forms  fertilization  does  occur  at 
some  period  in  the  life  cycle,  after  a  widely  variable  number 
of  parthenogenetically  produced  generations,  but  there  are  a 
few  Metazoa,  e.g.,  the  wasp,  Rhoditis,  and  the  Crustacea,  Cypris, 
Limnadia,  and  sometimes  Apus,  in  which  males  never  develop 
and  fertilization  is  therefore  entirely  unknown  (Weismann). 
In  some  of  the  parthenogenetic  Crustacea  the  nucleus  of  one 
of  the  polar  bodies  seems  to  act  as  a  fertilizing  nucleus  (see 
Chapter  IV),  so  that  a  sort  of  autogamic  process  occurs, 
recalling  that  of  some  of  the  Protozoa  and  perhaps  analogous 
with  it.  In  all  of  these  Metazoa  the  form  and  history  of  the 
parthenogenetic  ova,  the  occasional  presence  of  vestigial 


204  GENERAL  EMBRYOLOGY 

spermathecse,  and  other  similar  conditions,  clearly  indicate  that 
this  is  a  secondary  or  derived  condition,  a  special  adaptation, 
which  therefore  throws  but  little  light  upon  the  fundamental 
significance  of  the  fertilization  process;  this  proves  only  that 
fertilization  is  not  in  all  cases  a  necessary  antecedent  to  develop- 
ment, and  that  the  ova  may,  in  certain  cases,  contain  in  them- 
selves complete  developmental  mechanisms,  and  need  neither 
the  addition  of  sperm  structures  nor  the  special  form  of  stimu- 
lation afforded  by  the  entrance  of  the  sperm  cell. 

Instances  of  merogony,  or  "male  parthenogenesis, "  where 
egg  fragments  containing  no  nuclear  substance  develop  after 
penetration  by  a  spermatozoon  (Echinoderms),  also  show 
that  the  addition  of  egg  and  sperm  structures,  each  incom- 
plete in  itself,  is  not  a  necessary  feature  of  fertilization. 

It  may  truly  be  said  that  the  obviously  secondary  character 
of  normal  parthenogenesis  renders  the  phenomenon  of  little 
value  as  evidence  regarding  the  real  meaning  of  fertiliza- 
tion in  the  vast  majority  of  instances.  But  such  an  objec- 
tion cannot  be  brought  against  the  evidence  from  experimental 
parthenogenesis,  and  probably  the  clearest  evidence  upon  this 
phase  of  the  subject  is  that  of  " artificial"  or  induced  partheno- 
genesis. In  view  of  this  fact,  and  of  the  great  general  impor- 
tance of  the  subject,  we  may  consider  this  matter  rather  fully. 

The  eggs  of  many  animals,  belonging  to  many  different 
classes  and  phyla,  normally  requiring  to  be  fertilized,  may  be 
stimulated  to  begin  their  development  by  chemical  or  physical 
treatment.  Thus  the  ova  of  many  Coelenterates,  Echinoderms, 
Annulates,  Molluscs,  and  even  some  Chordata  (Teleosts, 
Amphibia),  may  be  induced  to  commence  their  development 
parthenogenetically  by  being  subjected  to  the  action  of  a 
great  variety  of  organic  and  inorganic  substances  in  solution, 
to  unusually  high  or  low  temperatures,  to  physical  shock,  or  to 
various  other  conditions. 

In  this  process  of  artificial  parthenogenesis  two  phases 
must  be  kept  separate,  first,  the  phenomenon  of  maturation,  in 
those  cases  where  this  is  not  completed  at  the  time  fertilization 
normally  occurs;  and  second,  the  phenomena  of  cleavage  and 


FERTILIZATION  205 

differentiation,  occurring  subsequently  to  maturation.  Cer- 
tain acids  and  some  other  substances  seem  to  have,  accord- 
ing to  Morse,  a  specific  effect  in  bringing  about  maturation, 
either  not  producing  cleavage  or  actually  inhibiting  it,  in 
which  latter  case  it  may  then  be  induced  by  other  treatment. 
The  precise  actions  upon  the  egg  of  those  chemicals  inaugu- 
rating cleavage  are  varied,  but  for  the  most  part  they  appear 
to  effect  certain  changes  in  the  egg  which  are  similar  in  nearly 
every  instance.  For  example,  according  to  Loeb,  who  is  the 
pioneer  in  this  important  work,  in  the  sea-urchin,  the  best 
result,  that  is,  the  closest  imitation  of  natural  fertilization,  is 
secured  by  treating  the  eggs,  first  with  a  solution  of  a  monobasic 
fatty  acid,  such  as  butyric  acid,  for  one  or  two  minutes.  In 
many  cases  the  butyric  acid  can  be  replaced  by  an  alkaline 
solution  of  equivalent  strength  or  by  a  solution  of  almost  any 
fat  solvent.  This  treatment  results  in  the  formation  of  an 
apparently  typical  fertilization  membrane.  Then  second,  the 
eggs  are  treated  for  some  minutes  or  hours  (sea-urchin  eggs, 
thirty  to  fifty  minutes  at  15°  C.)  with  a  hypertonic  sea  water, 
that  is,  sea  water  whose  osmotic  pressure  has  been  raised  about 
50  per  cent,  above  normal  by  the  addition  of  salts,  such  as 
sodium  chloride.  Finally  the  eggs  are  returned  to  normal  sea 
water  and  cleavage  then  follows  in  quite  the  usual  fashion. 

What  is  actually  accomplished  within  the  egg  by  such  treat- 
ment as  this  is  largely  conjectural.  Loeb  suggests  that  the 
process  may  be  as  follows.  The  mature  ovum  is  surrounded  by 
a  relatively  impermeable  surface  film  which  prevents  the 
oxidations  necessary  to  development.  The  butyric  acid  or 
similarly  acting  substance,  by  dissolving  certain  fatty  constitu- 
ents near  the  egg  surface,  frees  from  association  with  these, 
certain  other  osmotically  active  materials  which  then  form  the 
permeable  fertilization  membrane,  and  thus  rapid  oxidations 
are  permitted.  Loeb  believes  that  the  nuclear  substance 
possesses  a  catalyzer  wrhich,  in  the  presence  of  oxygen,  brings 
about  a  synthesis  of  nuclein,  one  of  the  chief  constituents  of 
chromatin,  and  that  this  synthesis  of  nuclein  is  the  chief 
chemical  action  of  the  segmenting  ovum.  This  process  of 


206  GENERAL  EMBRYOLOGY 

cleavage  once  started  in  the  right  direction,  leads  then  in  a 
perfectly  natural  and  normal  way  to  the  later  processes  of  de- 
velopment. Loeb  suggests  farther  that  the  spermatozoon  may 
contain  certain  substances,  enzymes,  which  form  within  the 
egg,  materials  capable  of  producing  effects  similar  to  these, 
and  that  herein  lies  the  natural  stimulating  effect  of  the 
spermatozoon. 

Many  facts  regarding  this  hypothesis,  both  pro  and  con,  have 
been  forthcoming  in  recent  years,  but  it  is  still  too  early  to  say 
how  closely  this  approximates  the  truth.  We  might  add,  how- 
ever, that  Masing  and  others  have  not  been  able  to  detect  any 
marked  synthesis  of  nuclein  such  a  Loeb  describes  during 
cleavage.  And  quite  recently  Conklin  has  determined  that  the 
synthesis  of  nuclear  substance  is,  in  Crepidula,  at  least  no 
greater  than  the  synthesis  of  cytoplasm. 

The  eggs  of  other  forms  are  more  successfully  treated  by  other 
methods;  and  each  may  have  a  particular  treatment  which  is 
most  effective.  But  in  very  many  of  the  instances  of  artificial 
parthenogenesis  the  essential  result  of  the  treatment  seems  to  be 
a  process  of  membrane  formation  accompanied  by  the  with- 
drawal of  fluids  from  the  egg.  This  has  led  to  the  suggestion 
(Loeb)  that  the  spermatozoon  acts  in  this  fashion,  for  it  is 
relatively  very  deficient  in  fluids,  and  upon  entering  the  egg 
reduces  to  some  extent  the  relative  fluidity  of  its  cytoplasm, 
thus  acting  as  a  stimulus. 

That  the  action  of  the  spermatozoon  is  not  specific,  and  that 
fusion  of  the  two  germ  nuclei  is  really  not  necessary  to  inaugu- 
rate development,  is  clearly  shown  by  the  fact  that  almost  any 
spermatozoon,  of  whatever  species,  that  can  gain  entrance  to 
an  ovum,  is  capable  of  initiating  development,  and  of  effecting 
the  apparently  normal  cleavage  of  the  ovum;  to  what  extent 
the  internal  processes  of  fertilization  and  cleavage  are  entirely 
normal  in  such  a  case,  we  shall  see  later;  suffice  it  to  say  here 
that  frequently  a  foreign  sperm  nucleus  remains  quiescent  and 
takes  no  part  in  the  formation  of  the  mitotic  figure. 

It  is  true  that  the  development  of  artificially  fertilized  ova 
seldom  proceeds  farther  than  the  cleavage  stages.  As  a  rule 


FERTILIZATION  207 

the  lower  the  organism  in  the  evolutionary  series,  the  farther 
its  development  may  proceed.  And  while  some  artificially 
fertilized  Echinoderm  eggs  have  been  carried  past  the  larval 
stage  (Delage),  the  Chordate  ovum  (Teleost,  Cyclostome, 
Urodele)  will  cleave  only  a  few  times.  This  indicates  clearly 
that  the  parallelism  between  natural  and  artificial  fertilization 
is  not  complete,  although  it  is  not  unlikely  that  ultimately  a 
form  of  treatment  may  be  found  which  will  produce  just  the 
same  result  as  normal  fertilization,  save  in  so  far  as  this  is  con- 
cerned in  the  inheritance  of  individual  characteristics. 

In  all  cases  of  artificial  parthenogenesis  the  cleavage  figures 

o 

are  essentially  normal  except  that  the  reduced  or  ^  number 

of  chromosomes  is  present  (exceptions  to  this  have  been 
reported  by  Tennent  and  Hogue,  and  others) ;  the  poles  of  the 
spindle  are  occupied  by  typical  centrosomes,  formed  anew  by 
the  substance  of  the  egg  after  the  disappearance  of  the  oocyte 
centrosomes  of  the  maturation  spindle.  This  is  also  true  re- 
garding the  centrosomes  of  normally  parthenogenetic  eggs. 
And  there  are  several  instances  known  where  the  specific  effect 
of  certain  reagents  or  external  conditions  is  the  formation,  out 
of  the  cytoplasm  of  the  ovum,  of  numerous  centrosomes, 
apparently  of  normal  structure  and  each  with  a  small  aster 
(Yatsu). 

To  summarize  the  evidence  from  parthenogenesis,  both  nor- 
mal and  artificial,  we  may  say  that,  among  the  Metazoa,  the 
ovum  contains  within  itself  a  mechanism  sufficiently  complete 
to  function  for  a  time  at  least,  although  the  spermatozoon, 
when  it  enters,  does  add  to  this  mechanism  and  supplies  some 
parts  not  present  in  the  egg;  these  parts  either  are  not  abso- 
lutely necessary  or  they  may,  under  certain  conditions,  be 
supplied  from  the  structure  of  the  ovum  in  the  absence  of  the 
sperm.  And  further,  while  the  egg  may  be  stimulated  to 
develop  by  means  other  than  the  entrance  of  the  sperm,  this 
is  normally  the  form  of  stimulus  which  inaugurates  the  series 
of  reactions  we  call  development.  Taking  this  view  of  fertiliza- 
tion the  formation  of  the  spermatozoon  is  a  means  of  insur- 


208  GENERAL  EMBRYOLOGY 

ing  the  properly  effective  form  of  stimulus,  which  might  other- 
wise be  lacking  in  the  environment  of  the  egg. 

Turning  now  to  the  evidence  which  the  Protozoa  offer 
regarding  the  relation  of  fertilization  and  reproduction,  we  may 
approach  more  closely  the  problem  of  the  fundamental  signifi- 
cance of  syngamy.  Nearly  all  the  known  life  histories  of  Pro- 
tozoa are  cyclic  in  character.  The  process  of  reproduction  by 
simple  fission  may  proceed  uninterruptedly  for  a  longer  or 
shorter  period,  but  finally  this  is  interrupted  by  some  form  of 
syngamy.  Formerly  this  seemed  obviously  to  mean  that  the 
ordinary  reproductive  processes  depended  ultimately  upon  a 
process  of  conjugation  or  fertilization,  and  that  the  life  cycle  in 
the  Protozoa  was  essentially  similar  to  that  of  the  Metazoa,  the 
divisions  of  the  somatic  cells  of  the  latter  being  equivalent  to  the 
simple  fissions  of  the  former,  and  the  fertilization  processes  of 
both  being  essentially  similar  (homologous) .  It  was  overlooked 
at  first  that  the  process  of  fertilization  might  just  as  well  be 
considered  the  result  of  vegetative  divisions  as  the  cause  of 
them;  to  this  phase  of  the  relation  we  shall  return  shortly. 

It  is  true  that  in  some  Protozoa,  e.g.,  Noctiluca,  Trichosphoer- 
ium,  some  Gregarines,  fertilization  is  really  followed  by  a 
marked  increase  in  reproductive  activity.  And  it  is  often  true 
that  multiple  fission  tends  to  follow  conjugation.  But  in 
other  cases  reproductive  activity  seems  not  to  be  affected  by 
fertilization.  And  in  many,  probably  most  Protozoa,  fertiliza- 
tion tends  to  inhibit  reproduction.  In  many  Rhizopods, 
Flagellates,  and  Ciliates,  a  pause  in  the  succession  of  fissions 
may  be  quite  marked  after  conjugation.  In  many  of  the  Sporo- 
zoa  a  period  of  encystment  follows,  and  the  same  is  true  of 
many  Algae.  In  such  cases,  therefore,  fertilization  seems 
opposed  to  reproduction,  or  at  least  to  any  immediately  ensuing 
processes  of  multiplication;  it  may  still  be  true  that  the  ultimate 
effect  of  fertilization  may  be  increased  rate  or  duration  of  fission. 
Conjugation  may  occur  without  reproduction;  reproduction 
may  occur  without  conjugation.  And  that  conjugation  is 
frequently  to  be  regarded  as  determined  by  external  rather  than 
internal  conditions  is  indicated  by  the  occurrence  of  so-called 


FERTILIZATION  209 

" epidemics"  of  conjugation  which  may  often  be  observed  in 
Protozoan  cultures.  In  such  cases  conjugation  may  often  be 
artificially  induced  by  regulating  the  character  and  amount  of 
the  food  supply  (Jennings). 

It  may  be  concluded,  therefore,  that  among  the  Protozoa  the 
processes  of  reproduction  and  fertilization  are  not  funda- 
mentally related,  and  the  primary  significance  of  fertilization 
must  be  sought  in  some  other  relation.  This  view  is  widely 
accepted  to-day  and  it  consequently  becomes  necessary  to 
explain  the  practically  universal  association  of  the  two  proc- 
esses among  the  Metazoa,  the  only  exceptions  being,  as  we 
have  seen  above,  the  secondary  and  obviously  derived  instances 
of  normal  parthenogenesis.  The  commonly  suggested  explana- 
tion is  the  following. 

Whatever  the  real  significance  of  fertilization  may  be,  it 
seems,  for  reasons  which  will  appear  later,  a  condition  for 
continued  existence  of  specific  forms  of  protoplasm  that 
occasionally  some  disturbance  of  its  inner  structure  should 
occur,  such  as  would  result  from  the  mingling  of  the  substances 
of  two  distinct  individuals.  Among  the  single-celled  organisms 
this  may  occur  at  any  time,  whenever  that  action  would  form  a 
natural  response  to  internal  conditions  of  the  organisms. 
Among  the  Metazoa,  on  the  other  hand,  such  a  complete  fusion 
of  cells,  and  particularly  of  nuclei,  can  occur  only  when  the 
organisms  are  in  the  form  of  single  cells,  i.e.,  gametes,  and 
differentiation  of  the  organism  is  at  a  minimum.  Thus  whereas 
the  two  processes  are  originally  distinct  and  unrelated  in  their 
origin  in  the  Metazoa,  they  have  come  to  be  related,  and  now 
fertilization  appears  as  the  first  step  in  reproduction. 

Another  general  hypothesis  regarding  the  function  and  signifi- 
cance of  fertilization  is  the  rejuvenation  hypothesis,  associated 
chiefly  with  the  names  of  Biitschli,  Maupas,  and  Richard  Hert- 
wig.  This  is  based  to  a  large  extent  upon  the  phenomena  of 
the  Protozoan  life  cycle.  It  involves  as  a  starting  point  the 
assumption,  partly  based  upon  observation,  that  protoplasmic 
activity  tends  gradually  to  diminish  in  intensity,  and  that 
associated  with  this  diminution  are  certain  morphological  altera- 


210  GENERAL  EMBRYOLOGY 

tions  in  the  structure  and  composition  of  the  cell.  Altogether 
these  modifications  are  known  as  senescence.  A  frequent 
characteristic  of  a  senescent  cell,  in  both  Protozoa  and  Metazoa, 
is  the  relatively  large  proportion  of  cytoplasm  as  compared 
with  nuclear  substance.  It  is  further  assumed  that  syngamy 
and  the  consequent  admixture  of  nuclear  and  cytoplasmic 
materials  of  two  individuals,  perhaps  representing  different 
races,  causes  the  restoration  of  the  senescent  protoplasm  to  a 
condition  of  vigor,  in  a  word,  brings  about  rejuvenation.  It 
would  follow  from  this,  that  protoplasmic  activity  is  cyclic, 
and  that  periods  of  senescence  would  be  followed  by  death  un- 
less fertilization,  or  an  equivalent  process,  should  occur. 

Precisely  what  is  involved  in  the  process  of  rejuvenation 
cannot  be  stated  definitely.  Richard  Hertwig  suggests  that 
senescence  is  due  chiefly  to  changed  nuclear-cytoplasmic  rela- 
tions resulting  from  repeated  cell-fissions,  and  that  in  rejuve- 
nation there  is  essentially  a  restoration  of  the  normal  nuclear- 
cytoplasmic  ratio,  as  well  as  a  certain  chemical  and  physical 
reorganization  of  the  protoplasm  through  the  combination  of 
materials  from  two  more  or  less  unlike  individuals.  Loeb  and 
others,  as  we  have  seen,  also  regard  the  rapid  synthesis  of 
nuclein  as  the  most  important  consequence  of  fertilization. 
Still  others  (Minot,  Bernstein)  believe  that  rejuvenation  is  not 
only  a  nuclear-cytoplasmic  phenomenon  involving  or  resulting 
from  an  increase  in  the  relative  amount  of  nuclear  substance, 
but  that  it  further  includes  an  increase  in  the  property  of  growth, 
i.e.,  the  formation  of  new  protoplasm,  both  nuclear  and  cyto- 
plasmic, out  of  non-living  substance. 

The  real  evidence  for  the  cyclic  character  of  the  life  processes 
of  the  Protozoa  is  chiefly  that  of  Maupas  and  Calkins,  who 
showed  that  in  Paramoecium  and  some  other  Ciliates,  when 
conjugation  is  prevented,  there  occur,  under  laboratory  condi- 
tions, periods  of  depression  in  vital  activity,  accompanied  by 
changes  in  structure,  i.e.,  periods  of  senescence.  This  depres- 
sion leads  finally  to  death  unless  conjugation  is  permitted,  or 
unless  the  organisms  are  subjected  to  some  form  of  stimulus. 
If  stimulated  by  chemical  or  physical  means,  or  naturally 


FERTILIZATION  211 

through  conjugation,  the  organisms  may  in  some  cases  recover 
their  original  vigor  and  begin  a  new  cycle  with  youth  renewed. 

But  rejuvenation  is  by  no  means  always  the  result  of  conjuga- 
tion, for  frequently  the  senescent  organisms  perish  in  spite  of 
conjugation;  and  it  may  even  be  the  case  that  the  descendants 
of  cells  which  have  conjugated  before  the  signs  of  senescence 
have  appeared,  perish  sooner  than  their  immediate  relatives 
of  approximately  the  same  age,  which  have  been  prevented  from 
conjugating.  Moreover,  Jennings  has  shown  that  in  certain 
races  of  Paramoecium  aurelia-caudatum  conjugation  may  occur 
at  intervals  of  only  one  or  two  weeks,  while  in  other  races  of 
the  same  species  conjugation  occurs  only  at  intervals  of  a  year 
or  longer,  and  in  still  a  third  race  no  conjugation  was  observed 
during  a  period  of  three  years,  although  during  this  time 
observation  was  not  so  continuous  as  to  preclude  the  possibility 
of  conjugation  having  occurred. 

Valuable  evidence  upon  the  question  of  the  cyclic  character 
of  the  Protozoan  life  history  is  afforded  by  the  work  of  Woodruff, 
who  has  shown  that  if  more  natural  conditions  are  substituted 
for  the  artificial  and  more  uniform  conditions  of  the  laboratory, 
no  cyclic  relation  appears,  in  some  strains  of  Paramoecium  at 
least.  By  continually  altering  the  character  of  the  food,  and 
by  imitating  in  other  ways  the  naturally  variable  conditions 
of  pond  life,  he  has  been  able  to  continue  a  single  race  of 
Paramcecium  for  over  five  years.  During  this  period  more 
than  3000  generations  were  formed  by  simple  fission,  and  in 
all  this  time  conjugation  did  not  occur,  and  no  periods  of 
depression  or  signs  of  structural  modification  could  be  ob- 
served.1 Finally,  Woodruff  has  been  able  to  carry  a  culture  of 
Paramoecium  on  a  uniform  diet  of  beef  extract,  which  is  sup- 
posed to  contain  all  of  the  materials  necessary  for  their  life, 
for  ten  months  (about  450  generations)  without  any  indication 
of  senescence. 

Such  facts  as  the  foregoing  show,  first,  that  protoplasmic 
activity  among  the  Ciliates  may  not  be  cyclic  in  character  under 

1  On  Sept.  27th,  1912,  Professor  Woodruff  writes  that  this  culture  is 
in  its  3265th  generation,  and  still  normal. 


212  GENERAL  EMBRYOLOGY 

certain  conditions,  and  second,  that  when  cyclic  periods  of 
protoplasmic  depression  do  occur  the  protoplasm  may  be 
restored  to  a  condition  of  normal  vigor,  either  by  physical  or 
chemical  stimuli,  or  by  fertilization.  Supposing,  and  the  sup- 
position is  highly  probable  though  not  completely  demon- 
strated, as  a  fact,  that  the  living  processes  do  tend,  in  the  absence 
of  continued  stimulation,  to  diminish  in  intensity  or  otherwise 
to  deviate  from  the  normal,  then  we  find  in  the  process  of  fer- 
tilization a  natural  means  of  insuring  the  receipt  of  stimuli 
which  might  otherwise  be  lacking.  The  onset  of  those  struc- 
tural and  physiological  modifications  called  senescence,  leads 
to  a  modification  in  the  behavior  of  the  organisms,  i.e.,  they 
form  gametes  and  conjugate. 

This  becomes  clearer  when  we  recall  that  life  itself  is  re- 
sponse— reaction  to  the  stimuli  resulting  from  changed  relations. 
Such  a  changed  relation  may  result  (a)  from  changes  in  both  the 
environment  and  the  cell  or  organism,  or  (6)  from  changes  in 
the  environment  alone  while  conditions  within  the  organism 
remain  comparatively  uniform,  or  (c)  from  changes  within  the 
organism  while  the  external  conditions  remain  comparatively 
uniform.  Of  these  three  possibilities  the  first  two  are  certainly 
the  most  frequent  in  the  lives  of  most  free-living  Protozoa. 
But  we  may  interpret  fertilization  as  fundamentally  a  means 
of  ensuring  a  changed  relation  through  the  realization  of  the 
third  possibility  in  the  absence  of  the  other  two.  In  a  way  the 
Ciliates  act  so  as  to  ensure  automatically  a  changed  relation 
between  organism  and  environment;  when  external  conditions 
become  too  uniform  to  bring  forth  the  normal  vegetative  ac- 
tivities, the  form  of  reaction  actually  changes  and  is  modified 
into  gamete  formation  and  fertilization,  which  immediately 
leads  to  an  internal  disturbance  and  the  condition  of  uniformity 
is  corrected,  whenever  it  may  occur. 

Among  the  Protozoa  we  find  this  division  of  labor  between 
vegetative  and  conjugative  or  fertilizing  cells  occurring  when- 
ever internal-external  relations  demand  it.  Among  the  Meta- 
zoa,  however,  such  a  division  of  labor  must  occur  at  a  certain 
period  in  the  life  history,  on  account  of  the  impossibility  of  the 


FERTILIZATION  213 

complete  fusion  of  two  whole  organisms  at  any  time  other 
than  when  they  are  in  the  form  of  single  cells;  consequently  we 
find  vegetative  and  gamete-forming  tissues  differentiated  side 
by  side,  and  since  these  are,  as  components  of  a  single  organism, 
in  a  fairly  constant  environment  removed  from  continuously 
rejuvenating  stimuli,  it  is  the  function  of  the  gamete-forming 
tissues  to  form  single  cells  which  can  fuse  with  cells  of  other 
individuals  and  thus,  by  altering  the  composition  of  the  organ- 
ism, alter  its  relation  to  external  conditions.  And  the  almost 
universal  association  of  reproduction  and  fertilization  consid- 
ered as  a  rejuvenative  process  among  the  Metazoa  may  have  an 
added  significance;  the  two  occur  together,  not  because  they 
are  directly  related  to  one  another,  but  because  they  are  both 
occasioned  by  the  same  condition,  or  rather  by  the  same  limi- 
tation of  opportunity,  for  the  complete  fusion  of  multicellular 
organisms  can  occur  only  when  these  are  hi  the  form  of  single 
cells,  or  gametes. 

Reference  to  the  third  possible  significance  of  fertilization 
may  be  more  brief  because  of  its  extremely  hypothetical  char- 
acter. The  idea  that  the  process  of  fertilization  is  primarily 
related  to  the  phenomena  of  variation  is  associated  chiefly  with 
the  names  of  Weismann  and  Oscar  Hertwig.  The  scanty  and 
uncertain  character  of  the  evidence  here  is  indicated  by  the 
fact  that  there  are  two  exactly  opposed  views  as  to  the  nature 
of  the  relation.  Hertwig  maintains  that  the  effect  of  fertiliza- 
tion is  to  limit  variation  within  a  species,  by  tending  to  bring 
back  to  the  normal,  through  the  process  of  heredity,  the  progeny 
of  extreme  fluctuations  and  unusual  or  abnormal  variations 
(mutations),  because  the  likelihood  of  their  mating  with  the 
much  more  common  mediocre  or  average  individuals  is  so 
much  greater  than  that  of  their  mating  with  their  likes.  Weis- 
mann, on  the  other  hand,  maintains  that  the  effect  of  syngamy 
or  " amphimixis"  is  to  cause  or  promote  variation,  which  would 
result  from  the  new  organic  combinations  in  the  continued 
admixture  of  the  gametic  nuclei  of  different  individuals.  In 
this  way  the  process  of  fertilization  becomes  of  great  evolu- 
tionary significance  in  that  it  accounts,  in  part  at  any  rate,  for 


214  GENERAL  EMBRYOLOGY 

the  presence,  indeed  the  origin,  of  variations  and  fluctuations, 
the  "raw  materials"  of  evolution. 

Both  of  these  views  are  based  upon  the  more  fundamental 
and  underlying  hypothesis  of  the  representative  particle  nature 
of  the  elements  of  the  chromosomes,  or  perhaps  of  other  portions 
of  the  germ  cells,  which  themselves  vary  in  their  structure  or 
their  combinations.  Here  again  the  occurrence,  among  the 
Metazoa,  of  fertilization  only  when  the  organisms  are  in  the 
form  of  single  cells,  grows  out  of  the  fact  that  complete  nuclear 
fusion  can  occur  only  when  in  this  state. 

There  is  little  direct  factual  evidence  for  or  against  these 
views,  either  one  of  which  can  be  maintained  upon  theoretical 
grounds.  In  a  few  cases  it  is  known  that  the  amount  of 
variability  is  not  significantly  different  among  sexually  (gamet- 
ically)  and  asexually  (parthenogenetically)  produced  indi- 
viduals of  the  same  species.  And  from  the  standpoint  of  more 
recent  studies  upon  heredity  and  variation  the  evidence  is 
chiefly  either  negative  or  opposed  to  the  idea  that  this  relation 
constitutes  an  important  element  in  the  origin  or  present  func- 
tion of  fertilization.  The  present  aspects  of  this  relation 
between  fertilization  and  variation  merge  in  the  larger  question 
of  the  relation  with  heredity  which  we  may  refer  to  next. 

Whatever  the  significance  of  fertilization  may  prove  to  have 
been  originally,  its  relation  to  the  phenomena  of  heredity  is 
to-day  undoubtedly  its  most  important  aspect,  at  any  rate 
among  the  Metazoa.  The  general  subject  of  the  relation  of  the 
structure  of  the  germ  cells  to  the  main  facts  of  heredity  is 
reserved  for  consideration  in  Chapter  VII,  but  we  should  point 
out  here  some  of  the  underlying  conditions  involved  in  the 
fundamental  fact  of  the  union  of  the  two  germ  cells  derived  in 
nearly  all  cases  from  two  different  individuals  of  the  same  group 
or  species. 

As  pointed  out  in  the  introductory  chapter,  the  germ  cells 
are  not  to  be  regarded  as  the  material  links  between  successive 
generations  of  specific  organisms,  for  organismal  specificity  is 
not  discontinuous,  but  continuous,  and  the  germ  cells  are  no 
less  specific,  no  less  the  organism,  than  are  the  mature  indi- 


FERTILIZATION  215 

viduals  producing  the  germ  cells  or  produced  by  them.  From 
the  standpoint  of  the  fertilization  process  and  of  heredity,  the 
essential  fact  is  not  that  the  zygote  develops  into  an  individual 
of  the  same  species  to  which  belonged  the  organisms  producing 
the  gametes,  for  in  parthenogenesis,  for  example,  specific  organ- 
isms are  produced  in  the  absence  of  fertilization.  The  signi- 
ficant fact  here  is  that  offspring  may  possess  some  of  those 
characteristics  which  are  the  individual  possessions  of  either 
of  the  parents.  On  the  whole,  offspring  inherit,  or  may  inherit, 
equally  from  both  parents,  and  such  a  possibility  must  depend 
upon  the  fact  that  the  zygote  is  composed  of  substances  or 
structures  derived  from  both  the  parent  organisms. 

Of  course  the  only  substance  of  the  zygote  which  is  derived 
in  equal  or  approximately  equal  parts  from  the  two  parents  is 
the  chromatic  portion  of  its  nucleus,  and  it  is  frequently  said 
that  therefore  it  must  be  the  nuclear  structures  of  the  germ  cells 
which  are  involved  in  this  fact  of  equal  biparental  inheritance. 
And  yet  the  fact  should  not  be  disregarded  that  the  sperm  does 
bring  into  the  egg  a  certain  though  indeed  a  small  amount  of 
cytoplasm.  The  fact  that  the  individual  parental  characters 
are  inherited  equally  does  not  necessarily  mean  that  all  non- 
individual  characteristics  are  thus  inherited,  for  all  of  the 
more  general  species  characters  are  common  to  both  parents, 
and  the  offspring  might  conceivably  inherit  these  wholly  from 
either  parent.  From  this  point  of  view  the  contribution  by  the 
ovum  of  practically  the  whole  of  the  cytoplasm  of  the  zygote 
might  have  at  least  two  meanings.  It  might  mean  that  the 
general  species  characters  of  the  offspring  are  determined  by 
the  structure  of  the  cytoplasm,  and  only  the  individual  traits 
by  the  nuclear  structures;  it  would  follow  from  this  that  the 
spermatozoon  takes  a  relatively  subsidiary  part  in  heredity. 
Or  it  might  mean  that  the  two  gametic  nuclei  are  from  the 
beginning  equally  involved  in  the  determination  or  direction  of 
development,  while  the  cytoplasm  of  the  ovum  merely  affords 
the  great  bulk  of  the  material  basis  for  this  development,  and 
is  itself  in  no  wise  involved  in  the  qualitative  determination  of 
either  specific  or  individual  characters  of  the  offspring.  It 


216  GENERAL  EMBRYOLOGY 

would  follow  from  this  that  the  two  germ  cells  are  of  more 
nearly  equal  importance  in  the  process  of  heredity. 

We  may  finally  conclude  from  all  of  the  foregoing  discussion 
that  little  can  be  very  definitely  asserted  regarding  the  real 
function  of  fertilization  now,  and  still  less  regarding  the 
original  significance  of  the  process.  Some  of  the  questions 
involved  here  are  to-day  the  most  interesting  and  important  of 
the  unsolved  problems  in  the  fields  of  Embryology  and  Biology. 

It  is  reasonably  clear  that  fertilization  is  not,  at  the  present 
time,  a  simple  process,  although  it  may  have  been  so  originally. 
Doubtless  there  has  been  an  evolution  both  of  the  process  and 
of  the  consequences  of  fertilization,  just  as  there  has  been  of  all 
organ  structure  and  organ  physiology.  .  Furthermore,  it 
seems  clear  that  the  various  possibilities  described  above,  as  to 
the  significance  of  fertilization  are  not  mutually  exclusive: 
fertilization  may  be  important  for  several  of  these  reasons,  even 
in  a  single  case,  and  probably  it  has  no  one  meaning  that  is 
exclusively  true.  It  is  quite  possible  that  normally,  among 
the  Metazoa  to-day,  the  spermatozoon  may  bring  about  in  the 
ovum  the  formation  of  centrosomes  which  do,  as  a  matter  of 
fact,  take  an  important  part  in  the  succeeding  cleavages  of 
the  zygote,  it  may  also  chemically  and  physically  stimulate 
the  ovum  to  develop  by  bringing  about  initial  changes  in  its 
chemical  or  physical  structure  or  organization,  it  may  at  the 
same  time  introduce  substances,  the  effect  of  which  is  " rejuve- 
nation" of  the  specific  protoplasm  apart  from  the  reproductive 
phenomena,  and  finally  the  structure  of  the  spermatozoon  which 
does  these  things  may  also  affect  the  course  of  development  so 
that  individual  characteristics  of  the  male  parent,  as  well  as 
of  the  female  parent,  may  appear.  And  to  say  that  the  result 
of  fertilization  is,  for  example,  rejuvenation,  need  not  mean 
that  it  is  not  also  a  stimulus  to  reproduction,  a  controlling  factor 
in  variation,  and  a  means  of  heredity. 

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SILVESTRI,  F.,  Ricerche  sulle  fecondazione  di  un  animale  a  spermatozoi 
immobili.  Rich.  Lab.  Anat.  Roma  e  altri  Lab.  Biol.  6.  1902. 

SOBOTTA,  J.,  Die  Reifung  und  Befruchtung  des  Eis  von  Amphioxus 
lanceolatus.  Arch.  mikr.  Anat.  50.  1897. 

TENNENT,  D.  H.,  and  HOGUE,  M.  J.,  Studies  on  the  Development  of 
the  Starfish  Egg.  Jour.  Exp.  Zool.  3.  1906. 

WEISMANN,  A.,  (See  ref.  Ch.  I.) 

WILSON,  E.  B.,  (See  ref.  Ch.  II.) 

WOODRUFF,  L.  L.,  The  Life  Cycle  of  Paramecium  when  subjected  to  a 
Varied  Environment.  Amer.  Nat.  42.  1908.  Two  Thousand 
Generations  of  Paramoecium.  Arch.  Protist.  21.  1911.  A  Sum- 
mary of  the  Results  of  certain  Physiological  Studies  on  a  Pedigreed 
Race  of  Paramcecium.  Biochem.  Bull.  1.  1912. 

YATSU,  N.,  The  Formation  of  Centrosomes  in  Enucleated  Egg-Frag- 
ments. Jour.  Exp.  Zool.  2.  1905. 


CHAPTER  VI 
CLEAVAGE 

THE  grosser  and  externally  visible  processes  of  development 
begin  with  the  cleavage  of  the  fertilized  ovum,  or  zygote.  The 
period  of  cleavage  therefore  may  be  regarded  as  the  second  of 
the  " grand  periods"  in  individual  history.  During  the  first 
general  period  occur  all  the  events  leading  up  to  and  including 
the  final  establishment  of  the  zygote,  a  single  cell,  but  a  new 
organism.  During  this  second  period  the  Metazoan  really 
becomes  made  up  of  many  cells. 

The  essential  process  underlying  many  of  the  varied  phenom- 
ena of  cleavage  is  a  process  already  familiar,  mitotic  cell 
division;  but  it  is  true  that  cell  division  continues  long  after  the 
cleavage  period  proper  is  terminated,  in  some  tissues  through- 
out the  life  of  the  organism.  And  as  we  shall  soon  see,  the 
process  of  cleavage  involves  a  great  deal  more  than  merely  a 
succession  of  cell  divisions. 

Certain  general  characteristics  of  the  mitoses  of  the  period 
of  cleavage,  or  segmentation,  of  the  zygote,  may  be  observed, 
but  it  is  difficult  to  state  precisely  wherein  these  cell  divisions 
differ  from  those  of  later  development.  Probably  the  most  sig- 
nificant characteristic  of  the  divisions  of  this  period  is  that  they 
are  rarely  at  random,  but  nearly  always  occur  in  an  orderly 
fashion,  according  to  a  definite  schema  or  plan,  which  is  quite 
fixed  for  each  species  or  larger  group,  and  which  involves  the 
entire  cell  community.  The  mitoses  of  cleavage  are  fre- 
quently very  unequal  and  the  daughter  cells  may  be  very  unlike, 
not  only  in  size,  but  further  as  regards  cytoplasmic  character 
and  the  nature  of  various  cell  inclusions,  which  may  be  distrib- 
uted dissimilarly  during  these  divisions.  After  the  first  few 
mitoses  the  blastomeres  may  not  divide  synchronously,  so  that 
the  regular  and  rhythmic  geometric  increase  in  their  number 

219 


220 


GENERAL  EMBRYOLOGY 


to  2,  4,  8,  16,  etc.,  is  very  rarely  continued  after  eight  or  sixteen 
cells  have  been  formed;  the  regularity  of  division  may  be  dis- 
turbed as  early  as  the  second  cleavage.  In  some  forms  (e.g., 
Echinoderms,  Godlewski)  the  nuclei  of  the  daughter  cells 
enlarge  considerably  after  each  division,  in  some  cases  perhaps 
even  to  the  original  size:  the  cell  bodies  fail  to  do  so.  The 
result  is  the  constant  increase  in  the  relative  size  of  the  cell 
nuclei;  in  other  words,  while  the  amount  of  cytoplasm  increases 
only  slightly  or  not  at  all  during  cleavage,  the  amount  of  nucleo- 


C 


FIG.  105. — Types  of  blastulse.  A.  Amphioxus  (coeloblastula).  B.  Petro- 
myzon.  After  von  Kupffer.  C.  Noturus  (Teleost)  (discoblastula).  D.  Clava 
(Hydroid).  After  Hargitt.  (Solid  type.)  a,  animal  pole;  c,  segmentation 
cavity  or  blastoccel;  p,  periblast  (a  non-cellular  protoplasmic  layer  resting  upon 
the  yolk  mass) ;  v,  vegetal  pole. 

plasm  increases  considerably,  so  that  at  the  close  of  this  period 
the  organism  contains  an  appreciably  greater  proportion  of 
nuclear  material  than  did  the  zygote.  This,  however,  may  not 
be  regarded  as  a  general  characteristic  of  the  cleavage  mitoses 
in  all  organisms  (Conklin). 

After  a  number  of  cells,  varying  in  different  species,  have  been 
formed  they  become  arranged  so  as  to  limit  an  internal  cavity 
filled  with  a  fluid.  In  its  simplest  and  apparently  most  primi- 


CLEAVAGE  221 

tive  or  typical  form,  the  figure  thus  esfablished  is  a  hollow 
sphere,  the  wall  of  which  is  composed  of  a  single  layer  of  cells 
or  blastomeres.  This  structure,  however  its  actual  form  may 
deviate  from  this  type,  is  termed  the  Uastula,  and  the  cavity 
within  is  the  blastocoel,  or  segmentation  cavity  (Fig.  105).  The 
diverse  forms  of  the  blastula  depend  immediately  upon  the 
arrangement  of  the  preceding  cell  divisions;  the  blastuia  may  in 
some  cases  be  almost  or  quite  solid,  so  that  the  blastoccel  exists 
only  virtually. 

About  the  time  the  blastula  is  formed  the  successive  cleav- 
ages have  reduced  the  cell  size  to  a  physiological  minimum  and 
thereafter  the  daughter  cells  increase  in  size  subsequently  to 
each  division,  and  there  is  no  further  reduction  in  the  size  of 
the  blastomeres ;  the  volume  of  the  cytoplasm,  as  well  as  of  the 
nucleoplasm,  commences  to  increase,  in  other  words  the  organ- 
ism begins  to  grow.  The  relative  time  of  the  appearance  of  this 
growth  phase  is  widely  different  in  different  forms ;  in  the  Echi- 
noids  it  appears  when  about  sixty-four  cells  have  been  formed 
(Godlewski).  While  there  is  no  general  and  externally  visible 
indication  marking  a  definite  close  of  the  cleavage  period,  the 
formation  of  some  type  of  blastula,  or  the  initiation  of  cyto- 
plasmic  growth,  is  more  or  less  arbitrarily  assumed  to  mark  its 
termination,  although  many  of  the  processes  characteristic  of 
this  phase  of  development,  including  of  course  cell  division, 
may  continue  for  some  time  longer.  We  may  now  define  cleav- 
age as  that  early  period  of  development  characterized  externally 
by  a  rapid  and  orderly  succession  of  mitoses,  which  results  in 
the  formation,  from  the  zygote,  of  a  regularly  arranged  group 
of  small  blastomeres  possessing  relatively  large  nuclei. 

In  most  cases  there  is  a  marked  tendency  for  the  blastomeres 
to  assume  a  spheroidal  form,  more  or  less  modified  by  the 
tension  with  which  the  cells  are  held  together,  by  the  pressure 
of  the  egg  membranes,  etc.  Sometimes  the  cells  round  up  so 
as  to  become  practically  spheres,  in  contact  with  one  another 
by  greatly  restricted  surfaces  (Amphioxus,  Echinoderms,  Ccelen- 
terates).  In  other  instances  (frog,  chick,  Arthropods)  the 
separations  between  the  blastomeres  are  mere  grooves  or  fur- 


222  GENERAL  EMBRYOLOGY 

rows  on  the  surf  ace  \)i  the  mass;  here  the  cells  are  broadly  in 
contact  and  in  some  instances  remain  connected  by  very 
delicate  protoplasmic  bridges  similar  to  those  connecting  tissue 
cells  previously  mentioned.  In  a  few  instances  (some  Arthro- 
pods, a  few  Coelenterates)  these  early  cell  divisions  may  be 
imperfect,  the  nuclei  alone  dividing  and  forming  a  syncytium; 
later  the  cytoplasm  also  divides  simultaneously  into  a  number 
of  complete  cells.  Or  cells  once  formed  may  fuse  into  syncytial 
masses,  as  in  some  Crustacea. 

The  internal  processes  of  development  occurring  during  this 
period  are  of  greater  importance  than  the  external  phenomena. 
As  stated  above  (Chapter  V)  one  of  the  underlying  processes 
of  great  importance  seems  to  be  the  synthesis  of  chromatin 
which  occurs  at  this  time. 

In  the  opinion  of  many  this  is  a  highly  characteristic  chemical  process 
of  early  development.  It  results  from  rapid  oxidations  within  the  egg 
which  were  made  possible  by  the  transformation  of  the  egg  membrane 
from  a  condition  of  relative  impermeability,  to  a  state  of  high  permea- 
bility, oxygen  thus  being  readily  admitted  from  without.  This  trans- 
formation is  thought  to  result  from  the  chemical  reactions  of  the 
cytoplasm  following  fertilization,  during  which  there  occurs  also  the 
activation,  or  perhaps  the  introduction,  of  specific  enzymes  which  bring 
about  this  characteristic  oxidative  synthesis. 

This  view  as  to  the  chemical  process  most  essential  in  cleavage 
agrees  well  with  the  "kern-plasma"  theory  of  Richard  Hertwig,  accord- 
ing to  which,  as  already  mentioned,  the  ovum  and  zygote  are  to  be 
regarded  as  instancing  abnormal  or  especially  adapted  relations  between 
nucleus  and  cytoplasm ;  for  here  the  relative  amount  of  cytoplasm  is  far 
in  excess  of  the  common  proportion.  In  cleavage,  with  its  proportional 
increase  in  nuclear  substance,  we  should  see  a  restoration  to  or  toward 

Volume 

the  normal  of  the  kern-plasma  ratio  ~-  — —  (77") 

Volume  \Vc] 

cytoplasm 

Another  internal  process  is  of  prime  importance.  We 
have  already  become  familiar  with  certain  facts — that  one 
result  of  maturation  and  fertilization  is  the  presence  in  the 
zygote  of  two  similar  chromosome  groups,  derived  respectively 
from  the  male  and  female  parents;  that  while  the  egg  nucleus 


CLEAVAGE  223 

nearly  always  returns  to  a  " resting"  stage  before  its  fusion 
with  the  sperm  nucleus,  the  latter  may  or  may  not  do  likewise 
before  the  fusion  of  the  two  germ  nuclei  into  the  cleavage  nu- 
cleus; that  the  egg  and  sperm  nuclei  may  or  may  not  form 

o 

separate  spiremes  each  with  ^  chromosomes  during  the  pro- 
phase  of  the  first  cleavage  figure  of  the  zygote;  and  finally  that 
whatever  the  preliminary  details  may  have  been,  the  constant 
and  essential  facts  regarding  this  first  cleavage  figure  are,  (1) 
that  the  chromosome  group  now  consists  of  the  full  somatic 
number  of  univalent  elements,  (2)  that  these  are  present  in 
pairs,  and  (3)  that  they  have  been  derived  equally  from  the 
two  parents. 

As  cleavage  begins  the  first  important  step  is  the  longitudinal 
division  of  each  chromosome;  the  halves  diverge  during  the 
anaphase  of  the  first  mitosis,  and  into  the  nucleus  of  each 
daughter  cell  there  passes  a  precisely  similar  group  of  s  chromo- 
somes, paired  as  before  and  derived  in  equal  numbers  from  each 
of  the  two  parents.  In  each  succeeding  mitosis  the  same  thing 
happens.  So  that  in  every  cell  of  the  blastula,  and  probably 
even  of  the  fully  matured  organism,  the  nucleus  is  composed  of 
substance  derived  in  equal  parts  from  the  male  and  the  female 
parents  (Fig.  106).  In  some  forms  (Copepods,  Ascaris)  the 
two  parental  chromosome  groups  appear  to  remain  fairly 
distinct  up  to  a  late  stage  in  cleavage  (Fig.  107).  And  in 
some  cases  of  hybridization,  when  the  chromosomes  of  the 
two  parents  are  easily  distinguished  by  differences  in  size  and 
form  (e.g.,  the  hybrids  of  Fundulus  and  Menidia  described  by 
Moenkhaus),  such  a  process  of  equal  distribution  of  the  chro- 
mosomes can  be  clearly  followed  into  the  blastula  stage  (Fig.  39). 
It  may  fairly  be  assumed,  therefore,  that  Huxley's  comparison 
of  the  body  of  an  organism  with  a  web,  of  which  the  warp  comes 
from  one  parent,  the  woof  from  another,  has  been  justified  by 
the  subsequently  discovered  facts  of  development.  This  idea 
has  been  spoken  of  as  the  "autonomy  of  the  male  and  female 
chromosome  groups."  It  follows  from  this  that  the  parental 
chromosome  groups  of  the  primordial  germ  cells  of  the  new 


224 


GENERAL  EMBRYOLOGY 


Z> 


FIG.  106. — Diagrams  illustrating  the  distribution  of  the  paternal  and  maternal 
chromosomes  during  cleavage.  A.  Zygote  containing  sperm,  d\  and  egg,  2  , 
pronuclei,  with  similar  chromosome  groups.  B.  First  cleavage  figure.  All  the 
chromosomes  on  the  spindle,  and  each  divided.  C.  Two-cell  stage,  each  nucleus 
containing  equivalent  chromosome  groups  of  paternal  and  maternal  origin. 
D.  Second  cleavage  figure;  the  first  figure  is  repeated  in  each  cell.  E.  Four-cell 
stage.  Nuclei  all  alike,  and  each  composed  of  similar  contributions  from  each 
parent. 


CLEAVAGE 


225 


organism  are  also  separate  and  remain  so  through  their  descend- 
ants, the  oogonia  and  oocytes,  or  spermatogonia  and  spermato- 
cytes,  of  the  mature  individuals,  until  their  period  of  synapsis, 
when  the  members  of  each  pair  of  chromosomes,  similar  but  of 
diverse  ancestry,  unite  forming  a  single  bivalent  chromosome 
which  is  represented  in  the  mature  ovum  or  spermatozoon 
finally  formed.  It  has  already  been  suggested  that  this  process 
of  synapsis,  the  ultimate  fusion  of  paternal  and  maternal  chro- 


A 


IB 


FIG.  107. — A.  Cleavage  figure  in  one  of  the  first  two  blastomeres  of  the  egg  of 
the  Crustacean,  Cyclops  strenuus,  showing  the  independence  of  the  paternal  and 
maternal  chromosome  groups.  After  Riickert.  B,  C,  D.  Primitive  germ  cells 
from  embryos  of  the  skate,  Raja,  showing  the  duplex  character  of  the  nuclei. 
B  and  C  are  from  a  stage  about  the  close  of  gastrulation ;  D  from  a  larva  of  10  mm. 
After  Beard. 

« 

mosomes,  may  be  regarded  as  the  final  step  in  syngamy,  and 
that  it  is  at  the  same  time  the  first  step  in  the  beginning  of  a 
new  organism. 

With  these  briefly  stated  introductory  facts  in  mind  we  may 
proceed  to  a  more  precise  description  of  the  events  of  the 
cleavage  period.  The  process  of  cleavage  may  be  described  in 
several  different  ways,  or  rather  from  several  different  view- 
points. We  may  group  these  all  under  two  heads  and  consider 
cleavage,  first,  as  a  morphological  process,  emphasizing  pri- 
marily the  forms  of  cleavage  and  describing  the  relation  of  the 
cleavage  planes  and  the  blastomeres  (a)  to  the  entire  zygote,  and 


226  GENERAL  EMBRYOLOGY 

(6)  to  each  other.  Then  second,  we  may  emphasize  chiefly 
the  physiological  aspects  of  cleavage  describing  the  relation  of 
the  cleavage  processes  (a)  to  the  structure  or  organization  of 
the  ovum  and  zygote,  (6)  to  the  later  stages  in  the  development 
of  the  mature  organism.  As  a  matter  of  fact  these  aspects  of 
cleavage  are  not  really  separate,  for  all  the  particular  phenom- 
ena of  cleavage,  as  of  development  in  general,  are  to  be  referred 
to  a  single  fundamental  condition,  namely,  the  organization 
of  the  ovum  or  zygote  as  it  is  related  to  external  conditions;  and 
if  our  knowledge  were  complete  here  we  should  be  able  to 
describe  all  the  phenomena  of  cleavage  from  a  single  viewpoint. 
But  for  the  present  we  shall  find  it  more  convenient  as  well  as 
more  instructive  to  separate  more  or  less  arbitrarily  and  to 
consider  apart,  the  chiefly  morphological  and  the  chiefly  physio- 
logical aspects  of  this  process. 

In  considering  the  relation  of  cleavage  to  the  grosser  struc- 
ture of  the  zygote  we  find  that  one  of  the  primary  factors  in 
determining  the  form  of  cleavage  is  the  relative  amount  and  the 
form  of  distribution  of  the  yolk  and  other  deutoplasmic  sub- 
stances contained  in  the  ovum.  In  Chapter  III,  three  types  of 
eggs  were  described  on  this  basis:  (1)  homolecithal  or  isolecithal 
(alecithal),  containing  little  deutoplasm,  distributed  with 
considerable  uniformity  throughout  nearly  the  entire  ovum: 
(2)  telokcithal,  containing  varying,  often  considerable  amounts 
of  deutoplasm  chiefly  localized  toward  the  vegetative  pole  of 
the  ovum;  (3)  centrolecithal,  really  a  form  of  telolecithal  ova 
in  which  the  deutoplasm  has  a  central  rather  than  a  polar 
localization. 

Corresponding  in  a  general  way  with  these  variations  in  yolk 
distribution  we  may  distinguish  certain  types  of  cleavage,  each 
however  with  certain  variations  which  may  sometimes  appear 
as  connecting  intergradations.  First  we  may  distinguish 
complete  and  incomplete  cleavage.  When  the  eggs  are  com- 
paratively small  and  of  the  homolecithal  type,  the  earlier 
cleavage  planes  pass  completely  through  them  in  meridional 
and  latitudinal  planes.  Theoretically  the  simplest  form  of 
complete  cleavage  is  that  known  as  equal  cleavage,  where  the 


CLEAVAGE  227 

egg  and  the  blast omeres  formed  from  it  are  always  divided 
equally,  so  that  the  constant  result  is  a  group  of  similar  cells. 
This  is  rarely  if  ever  completely  realized;  the  nearest  approach 
to  it  is  seen  in  the  Holothurian,  Synapta  (Fig.  108).  In  most 
examples  of  so-called  equal  cleavage  slight  inequalities  may  be 
detected  even  as  early  as  the  two-cell  stage  (Amphioxus),  and 
quite  frequently  in  the  four-,  or  eight-cell  stages.  This  modifi- 
cation of  equal  cleavage  is  known  as  adequal.  Amphioxus 
and  some  of  the  Echinoderms  (Fig.  109)  illustrate  the  fact  that 
no  sharp  distinction  can  be  drawn  between  equal  and  unequal 
cleavage,  for  equal  cleavage  soon  becomes  unequal,  and  the 
transition  appears  gradually. 

More  frequently  the  cleavage  though  still  total  is  distinctly 
unequal,  at  least  by  the  time  eight  cells  are  formed,  and  often 
from  the  very  beginning  of  cleavage.  This  is  characteristic 
of  those  telolecithal  eggs  in  which  the  accumulation  of  yolk  is 
slightly  or  moderately  marked,  as  in  most  of  the  Platyhelmin- 
thes,  Nemathelminthes,  Annulata,  Trochelminthes,  Mollusca, 
Ganoids,  and  Amphibia  (Figs.  110,  111).  This  leads  to  a  second 
general  type  of  cleavage,  the  incomplete  type,  where  a  portion 
of  the  ovum  remains  uncut  by  the  cleavage  planes  (Fig.  113). 
Such  eggs  are  known  in  general  as  meroblastic,  in  distinction 
from  the  holoblastic  ova  whose  cleavage  is  complete.  In 
telolecithal  eggs  with  very  large  accumulations  of  deutoplasm, 
the  cleavage  planes  are  nearly  restricted  to  the  protoplasmic 
region  and  extend  only  a  short  distance  out  into  the  yolk; 
this  is  known  as  partial  cleavage.  When  the  protoplasmic  part 
is  quite  definitely  restricted  to  the  animal  pole,  cleavage  is  of 
an  extremely  incomplete  type  known  as  discoid  (Fig.  116),  and 
the  result  is  the  formation  of  a  small  cap  or  disc  of  cells  on 
the  surface  of  the  yolk  mass  (Teleosts,  Reptiles,  Birds).  Or, 
if  the  egg  is  of  the  centroleciihal  type,  cleavage  is  limited  to  the 
peripheral  protoplasmic  layer  and  is  known  as  superficial 
(most  Arthropods)  (Figs.  117,  118). 

Since  there  are  all  intermediate  conditions  between  homo-, 
telo-,  and  centrolecithal  types  of  ova,  we  find,  as  we  should 
expect,  all  corresponding  intermediate  conditions  between  these 


228 


GENERAL  EMBRYOLOGY 


various  forms  of  cleavage;  the  details  are  not  particularly  in- 
structive and  may  be  omitted. 

We  may  now  turn  to  the  morphological  description  of  the 
relations  of  the  blastomeres  among  themselves.  Before 
describing  the  various  relations  which  these  may  exhibit,  it  will 
be  useful  to  describe  briefly  a  simple  form  of  total  and  equal 
cleavage,  which  may  be  regarded  as  a  typical  form.  Such  a 
form  of  cleavage  is  indeed  rare  but  it  is  found  in  the  homoleci- 
thal  and  holoblastic  egg  of  the  sea-cucumber,  Synapta,  as 


FIG.  108. — Cleavage  in  the  Holothurian,  Synapta,  Slightly  schematized. 
From  Wilson,  "Cell,"  after  Selenka.  A-E.  Two-,  four-,  eight-,  sixteen-,  and 
thirty-two  cell  stages.  F.  Blastula  of  128  cells.  B,  in  polar  view,  others  in  side 
view. 

described  by  Selenka  (Fig.  108).  The  earlier  cleavage  planes 
always  appear  in  a  definite  relation  to  the  polar  structure  of  the 
ovum,  and  they  are  described  as  if  the  main  axis  of  the  egg  were 
in  a  vertical  position.  The  first  cleavage  plane  passes  through 
both  poles  and  the  chief  axis  of  the  egg,  dividing  it  equally  and 
appearing  on  the  surface  as  a  complete  meridian.  The  second 
plane  is  similarly  meridional  or  vertical,  is  at  right  angles  to  the 
first,  and  divides  the  egg  into  four  equal  quadrants.  The  third 
division  plane  is  at  right  angles  to  the  first  two  and  is  therefore 
horizontal.  In  Synapta  it  is  practically  midway  between  the 


CLEAVAGE  229 

poles  of  the  egg  and  is  therefore  described  as  equatorial.  In 
most  ova  cleaving  according  to  this  general  rule,  the  third  plane 
is  displaced  a  variable  distance  above  the  equator  and  is  then 
termed  latitudinal.  When  equatorial  this  cleavage  divides  the 
four  equal  cells  into  eight,  again  equal,  arranged  in  upper  and 
lower  groups  of  four,  known  as  the  upper  and  lower  quartets. 
The  fourth  cleavage  is  again  meridional  and  is  really  double, 
for  two  planes  appear  simultaneously  dividing  each  pair  of 
opposites  in  each  quartet  similarly;  this  results  in  the  formation 
of  upper  and  lower  octets.  The  fifth  cleavage  is  horizontal  or 
latitudinal,  and  is  again  double  for  it  divides  simultaneously 
the  upper  and  lower  octets  each  into  two  horizontal  groups  of 
eight  cells,  so  that  the  ovum  is  nowr  divided  into  eight  vertical 
rows  of  four  cells  each.  The  cleavages  continue  to  alternate 
meridionally  and  vertically,  until  about  the  ninth  cleavage  when 
512  cells  are  formed.  After  this,  and  in  fact  usually  before  this 
time,  the  synchronism  of  cleavage  begins  to  be  disturbed,  some 
of  the  cells  dividing  more  rapidly. 

One  of  the  very  frequent  causes  of  departure  from  this  simple 
schema  is  the  telolecithal  character  of  the  ovum.  Here  the 
upper  quartet  is  usually  smaller  than  the  lower  and  the  fourth 
cleavage  appears  earlier  in  the  upper  quartet  or  cells  of  the 
animal  pole.  This  leads  very  soon  to  an  irregularity  in  the 
rhythm  of  cleavage,  which  may  be  entirely  lost  after  eight  or 
sixteen  cells  are  formed. 

This  typical  outline  of  cleavage  serves  as  a  basis  to  illustrate 
certain  "laws"  of  cleavage  which  may  be  referred  to  briefly 
at  this  point,  although  their  applicability  is  now  known  to  be 
very  limited.  The  first  of  these  is  the  Sachs-Hertwig  law 
describing  the  geometric  relations  of  the  successive  cleavage 
planes.  This  law  really  consists  of  two  parts  which  may  be 
stated  as  follows:  (1)  The  nucleus  of  a  blast omere  (or  of  any 
cell)  tends  to  assume  a  position  near  the  center  of  the  proto- 
plasmic mass.  From  this  results  the  equal  division  of  the  cell, 
provided  it  is  free  from  deutoplasm,  or  its  unequal  division 
if  the  cell  contains  deutoplasm  not  uniformly  distributed,  for 
in  the  latter  case  the  center  of  the  protoplasmic  mass  does  not 


230  GENERAL  EMBRYOLOGY 

correspond  with  the  center  of  the  entire  cell.  (2)  The  chief 
axis  of  the  mitotic  figure  tends  to  lie  in  the  longest  axis  of  the 
protoplasmic  mass.  The  result  of  this  is  that  in  cells  that  are 
approximately  spherical  and  homogeneous  with  respect  to  yolk 
content,  successive  cleavage  planes  tend  to  alternate  at  right 
angles  with  one  another,  for  it  would  always  be  the  longest  axis 
of  the  cell  that  is  divided,  and  in  most  cases  any  other  axis 
would  be  greater  than  one-half  the  longest  and  no  two  successive 
spindles  would  be  parallel.  The  regular  alternation  of  cleav- 
age planes  probably  depends,  as  a  matter  of  fact,  upon  a  more 
fundamental  relation,  namely,  the  position  of  the  centrosome. 
At  the  conclusion  of  a  mitosis  the  centrosome  lies  at  one  end 
of  the  axis  passing  perpendicularly  to  the  plane  of  division; 
when  the  centrosome  divides,  its  halves  usually  migrate  sym- 
metrically to  opposite  sides  of  the  nucleus,  occupying  the  poles 
of  an  axis  lying  parallel  with  the  plane  of  the  preceding  division, 
and  since  division  always  occurs  at  right  angles  to  the  axis 
connecting  the  centrosomes,  the  plane  of  one  division  will  be 
at  right  angles  to  that  of  the  preceding  or  succeeding  cleavage 
(Fig.  24).  Any  other  relation  between  successive  cleavage 
planes  involves  either  a  change  in  the  relative  position  of  the 
centrosomes,  or  a  rotation  of  the  cleavage  spindle  after  its 
formation.  Thus  in  the  formation  of  a  simple  epithelium, 
where  successive  cleavages  are  nearly  parallel,  the  centrosomes 
migrate  through  approximately  90°  at  some  time  during  the 
interkinesis;  and  in  the  cleavage  of  some  ova  encased  in  com- 
paratively rigid  shells,  the  position  of  the  spindle  may  change 
(Lepas,  Bigelow). 

Balfour's  law  of  cleavage,  which  is  really  a  corollary  of 
the  first  part  of  the  Sachs- Hertwig  laws,  concerns  the  rate 
rather  than  the  geometrical  relations  of  cleavage.  This  law 
states  that  the  rate  of  cleavage  is  inversely  proportional  to  the 
amount  of  deutoplasm  contained  within  the  cell.  It  follows 
from  the  fact  that  the  nucleus  tends  to  lie  in  the  center  of  the 
protoplasmic  mass,  that  in  the  unequal  division  of  cells  con- 
taining localized  deutoplasm,  the  smaller  cell  will  contain 
relatively  a  smaller  proportion  of  yolk  than  the  larger  cell,  and 


CLEAVAGE  231 

consequently,  being  free  from  the  influence  of  the  dead  and 
inert  deutoplasm,  will  be  able  to  divide  sooner. 

While  these  laws  are  often  applicable  to  the  processes  of  cleav- 
age in  a  general  way,  the  exact  study  of  cleavage  in  a  great 
variety  of  forms  has  disclosed  very  numerous  exceptions  and 
contradictions.  On  the  whole  we  may  say  that  such  laws,  though 
still  retaining  a  limited  applicability,  are  chiefly  interesting  as 
indicating  the  attempt  to  refer  the  phenomena  of  cleavage  to 
the  grosser  mechanical  relations  of  cell  structures.  It  is  now 
clear,  as  we  shall  see  later,  that  other  factors  are  of  greater  im- 
portance in  determining  the  form  and  rhythms  of  cleavage. 
The  fundamental " organization"  of  the  ovum,  which  is  not  only 
morphological  but  physiological  as  well,  is  the  primary  factor  in 
determining  the  characteristics  of  cleavage.  The  numerous 
" exceptions"  to  these  laws  of  cleavage  are  definitely  related  to 
both  the  organization  of  the  ovum  and  also  to  the  structural 
and  functional  characters  of  the  later  stages  of  development, 
since  these  too  are  primarily  determined  by  the  same  organiza- 
tion factor. 

Most  of  the  conditions  which  form  exceptions  to  these  rules, 
and  are  therefore  deviations  from  the  simple  and  regular 
form  of  cleavage  like  that  of  Synapta,  may,  following  Wilson 
("The  Cell,"  etc.),  be  grouped  under  three  heads.  (1)  Unequal 
Division.  While  this  is  usually  related  to  differences  in  deuto- 
plasmic  content,  there  are  many  instances  where  no  such  rela- 
tion can  be  made  out  and  the  inequalities  must  be  explained 
upon  other  grounds  (e.g.,  the  micromeres  of  the  Echinoids,  the 
teloblasts  of  certain  Annelids  and  Molluscs).  (2)  Cell  Dis- 
placement. This  may  result  from  the  atypical  position  of  the 
spindle  or  from  the  shifting  of  blastomeres  after  they  have  been 
formed.  Often  the  individual  blastomeres  are  only  lightly 
held  together  since  they  normally  show  a  tendency,  often  very 
marked,  to  assume  a  spherical  form.  Under  these  conditions 
they  might  tend  to  assume  a  position  described  by  the  law 
(Plateau's)  of  " least  surfaces"  or  " minimal  contact,"  according 
to  which  a  group  of  elastic  spheres,  like  bubbles,  held  together 
and  yet  free  to  move,  tend  to  become  arranged  in  such  a  way 


232  GENERAL  EMBRYOLOGY 

as  to  reduce  their  exposed  surfaces  to  a  minimum.  But  there 
are  frequent  exceptions  here  as  in  the  case  of  the  other  "laws" 
of  cleavage.  Furthermore,  the  active  migration  of  blastomeres 
is  not  infrequent,  so  that  cells  may  ultimately  be  found  in 
regions  considerably  removed  from  the  place  of  their  formation 
(Rotifers,  Molluscs).  (3)  Rhythm.  The  rate  of  division  fre- 
quently does  not  correspond  with  the  relative  amount  of 
deutoplasm.  The  factors  regulating  the  rhythm  of  division 
still  remain  largely  unknown.  It  is  true  here  as  in  many  other 
"exceptional"  cleavage  phenomena,  that  the  deviation  is 
related  to  the  future  morphological  or  functional  character 
of  the  developing  organism  or  of  parts  of  it.  We  shall  return 
to  this  aspect  of  cleavage  later. 

With  these  general  considerations  in  mind  we  may  proceed 
now  to  a  more  exact  description  and  classification  of  the  geo- 
metric forms  of  cleavage.  Here  we  shall  find  illustrations  of 
many  of  the  preceding  statements.  Considering  first  the  various 
forms  of  complete  cleavage  (holoblastic  ova),  we  may  distin- 
guish rather  roughly,  four  types,  radial,  spiral,  bilateral, 
irregular. 

(1)  Radial. — This  is  the  form  exemplified  by  Synapta  (Fig. 
108),  already  described  as  being  geometrically  the  simplest. 
This  should  perhaps  better  be  termed  rotatorial  than  radial,  for 
while  the  blastomeres  are  arranged  in  symmetrical  fashion  in 
any  single  plane  perpendicular  to  the  main  axis  of  the  egg, 
there  are  usually  considerable  differences  in  size  between  the 
cells  of  the  animal  and  vegetal  poles.     Cleavage  of  this  type 
is  found  in  the  sponges,  jelly-fishes,  and  many  Echinoderms, 
in  some  Nematodes  and  Rotifers.     In  the  sea-urchins  (Fig.  109) 
the  third  cleavage  is  meridional  in  the  upper  quartet,  in  the 
lower  latitudinal  and  very  unequal,  cutting  off  a  quartet  of  very 
small  cells  or  micromeres  which  curiously  are  found  at  the  lower 
or  vegetative  pole. 

(2)  Spiral. — This  may  be  regarded  as  a  modification  of  the 
radial  type  resulting  from  the  displacement  of  cells  so  that  the 
blastomeres  above  and  below  any  horizontal  cleavage  furrow 
tend  to  alternate  with  one  another  in  a  vertical  direction,  some- 


CLEAVAGE 


233 


FIG.  109. — Cleavage  in  the  sea-urchin,  Strongylocentrotus  lividus.  From 
Jenkinson,  after  Boveri.  Animal  pole  uppermost  in  all  cases,  a.  Primary 
oocyte  surrounded  by  jelly,  and  containing  large  germinal  vesicle  with  nucleolus. 
Pigment  uniformly  distributed  over  surface,  b.  Ovum  after  formation  of  polar 
bodies.  Pigment  forms  a  band  below  the  equator,  c,  d.  First  cleavage,  e. 
Eight-cells.  Pigment  almost  wholly  in  lower  quartet  (vegetative  blastomeres) . 
/.  Sixteen-cells.  The  lower  quartet  has  divided  latitudinally  and  unequally, 
forming  four  micromeres  at  the  vegetal  pole;  the  upper  quartet  has  divided  me- 
ridionally  forming  a  plate  of  eight  cells,  g.  Section  through  blastula.  h.  Later 
blastula,  showing  formation  of  mesenchyme  at  lower  pole,  t,  j,  k.  Three  stages 
in  gastrulation,  showing  the  infolding  of  the  pigmented'cells  to  form  the  endoderm 
(archenteron) .  In  j  the  primary  mesenchyme  is  separated  into  two  masses,  in 
each  of  which  a  spicule  is  formed  (k).  In  k  the  secondary,  or  pigmented,  mesen- 
chyme is  being  budded  off  from  the  inner  end  of  the  archenteron. 


234 


GENERAL  EMBRYOLOGY 


what  like  the  bricks  in  a  wall  or  the  bones  of  the  wrist.  They 
may  be  cut  off  in  this  way  on  account  of  the  obliquity  of  the 
spindle  in  the  parent  cell,  or  they  may  shift  to  this  position 
after  having  been  formed  according  to  the  radial  plan.  This 
spiral  arrangement  may  be  foreshadowed  in  the  four-cell  stage 
by  the  meeting  of  the  first  two  cleavage  planes  at  the  poles  of 
the  egg  in  the  form  of  a  zig-zag  line  instead  of  at  a  common 
point. 


C  D 

FIG.  110. — Cleavage  in  the  Annulate,  Polygordius.  From  Wilson,  "Cell." 
A,  B.  Four-  and  eight-cell  stages,  from  the  animal  pole.  C.  Side  view  of  eight- 
cell  stage.  D.  Side  view  of  sixteen-cell  stage. 

One  of  the  simplest  illustrations  of  this  type  is  the  adequal 
cleavage  of  Polygordius  (Fig.  110)  but  it  is  also  well  represented 
by  the  markedly  unequal  cleavage  of  many  Platyhelminthes, 
Nemertines,  Annelids,  and  Molluscs  (Figs.  Ill,  112,  119). 
These  forms  illustrate  at  the  same  time  a  graduated  series  in  the 
inequality  of  the  blastomeres.  This  inequality  may  appear  in 


CLEAVAGE 


235 


different  stages;  in  the  very  first  division  of  the  egg  (Nereis)] 
at  the  second  (ClaveUna) ',  third  (Cerebratulus,  Fig.  112);  fourth 
(sea-urchin),  or  still  later  (Synapta).  The  direction  which  the 
spiral  takes  is  fixed  in  each  species;  it  is  described  as  dextral 


FIG.  111. — The  eight-cell  stage  of  four  animals  showing  gradations  in  the 
inequality  of  the  third  cleavage,  and  in  the  extent  of  the  spiral  rotation  of  the 
micromeres.  From  Wilson,  "Cell."  All  viewed  from  the  animal  pole.  A.  The 
leech  Clepsine  (Whitman).  B.  The  chaetopod  Rhynchelmis  (Vejdovsky). 
C.  The  lamellibranch  Unio  (Lillie).  D.  Amphioxus. 

(dexiotropic)  or  sinistral  (keotropic)  when  the  upper  cells  are 
rotated  clockwise  or  counter-clockwise  respectively,  as  viewed 
from  the  animal  pole. 

(3)  Bilateral. — In  this  form  we  see  a  second  modification 
of  the  radial  type,  which  is  first  indicated  by  the  fact  that  the 
third  of  the  meridional  cleavages  fails  to  reach  precisely  the 
poles  of  the  egg  but  meets  either  the  first  or  second  plane  at 
some  distance  from  the  pole.  In  this  way  it  comes  about  that 


236 


GENERAL  EMBRYOLOGY 


FIG.  112. — Cleavage  in  the  Nemertean,  Cerebratulus  marginatus.  From 
Korschelt  and  Heider,  after  Zeleny.  X  216.  A,  B.  Two-  and  four-cell  stages 
in  side  view.  C.  Four-cells,  from  animal  pole.  D.  Eight-cells  from  vegetal 
pole.  E.  Eight-cells,  side  view.  F.  Sixteen-cells,  side  view.  G.  Twenty-eight 
cells,  side  view.  H.  Twenty-eight-cells  from  vegetal  pole.  For  explanation  of 
lettering,  see  p.  248. 


CLEAVAGE 


237 


one  of  the  first  two  cleavage  planes,  usually  the  first,  becomes 
the  plane  of  a  bilateral  symmetry  which  may  remain  quite  pro- 
nounced for  some  time,  and  indeed  corresponds  with  the  median 
plane  of  the  bilaterally  symmetrical  adult.  The  bilateral  type 


FIG.  113. — Meroblastic  cleavage  in  the  squid,  Loligo  pealii.  A,  B.  Egg  viewed 
obliquely,  showing  animal  pole.  X  45.  After  Watase.  C,  D.  Surface  views  of 
animal  pole,  more  highly  magnified,  to  show  bilateral  arrangement  of  blasto- 
meres.  From  Wilson,  "Cell,"  after  WatasS.  A.  Four-cell  stage.  B.  About 
sixty-cells.  Cells  at  animal  pole  very  small,  lowermost  cells  incomplete,  cell 
walls  extending  down  toward  the  uncleaved  lower  pole.  C.  Eight-cell  stage. 
D.  The  fifth  cleavage  (sixteen  to  thirty-two  cells),  a-p,  marks  the  plane  of 
the  first  cleavage  and  the  median  plane  of  the  organism;  l-r,  marks  the  second 
cleavage,  and  the  transverse  plane  of  the  organism. 

of  cleavage  is  found  in  the  Cephalopods  (Fig.  113),  a  few 
Rotifers  and  Nematodes,  in  Amphioxus  and  the  Ascidians,  and 
perhaps  in  most  of  the  Craniates,  but  in  these  last  forms  varia- 
tions are  more  frequent,  especially  in  those  forms  with  discoid 
cleavage. 


238 


GENERAL  EMBRYOLOGY 


A  rather  special  form  of  bilateral  cleavage  known  as  the 
disymmetrical  type  is  found  in  the  Ctenophores.  In  these 
Ccelenterates  the  first  and  second  furrows  are  meridional,  the 
planes  are  complete,  and  divide  the  egg  adequally.  The 
third,  a  double  cleavage,  is  oblique  to  the  first,  passing  in  the 
same  general  direction  as  this,  on  each  side  of  it,  but  approach- 


FIG.  114. — Diagrammatic  representation  of  the  cleavage  in  the  Ctenophore 
(based  upon  Beroe).  After  Ziegler.  A.  Four-cells,  from  side  and  above. 
B.  Eight-cells  in  side  view  (the  animal  pole  is  downward  here,  and  in  D).  C. 
Eight-cells,  from  animal  pole.  D.  Sixteen-cells,  from  side.  E.  Sixteen-cells, 
from  animal  pole,  ra,  ra.  Median  plane;  t,  t,  transverse  plane. 

ing  the  vegetal  pole  more  closely  than  the  animal.  The  descend- 
ants of  each  of  the  first  two  cells  then  become  symmetrically 
arranged  about  the  second  plane  so  that  two  similar  groups  of 
cells  are  formed,  each  group  bilaterally  symmetrical  with  refer- 
ence to  a  plane  perpendicular  to  the  plane  of  symmetry  of  the 
entire  cell  group  (Fig.  114). 


CLEAVAGE 


239 


(4)  Irregular. — In  many  phyla,  scattered  forms  are  known 
in  which  cleavage  adheres  to  no  single  or  simple  type  and  may 
truly  be  said  to  be  irregular.  This  does  not  mean  that  no 
definite  plan  is  followed,  for  each  species  follows  a  fixed  rule; 
these  cleavage  forms  are  in  this  sense  regular,  but  cannot 


FIG.  115. — Irregular  cleavage  in  the  Turbellarian,  Mesostomum  ehrenbergi. 
After  Bresslau.  X  700.  A.  Three-cell  stage,  in  section.  B.  Four-cells 
becoming  five.  Side  view.  C.  Seven-cell  stage,  from  animal  pole.  D.  Twelve- 
cells.  A.  Macromere,  giving  rise  to  A\  and  At  in  the  seven-cell  stage.  B.  First 
micromere,  forming  Bi  and  Bi  in  the  five-cell  stage.  C.  Second  micromere 
formed  in  three-cell  stage,  and  giving  rise  to  Ci  and  Cz  in  the  seven-cell  stage. 


be  described  in  general  terms.  We  cannot  stop  to  describe 
any  of  these  instances  in  detail.  Irregular  cleavage  may  be 
found  among  the  Porifera,  Coelenterates,  Platyhelminthes, 
Molluscoids,  Enteropneusta,  and  Teleosts;  a  typical  example 
is  illustrated  in  Fig.  115. 

The  remaining  forms  of  cleavage  are  grouped  as  incomplete^ 
and  are  found  among  those  species  with  markedly  telolecithal 


240 


GENERAL  EMBRYOLOGY 


or  centrolecithal  ova  (meroblastic).  Here  little  or  no  geometric 
regularity  of  cleavage  pattern  can  be  made  out.  We  may  add 
a  few  details  concerning  discoid  and  superficial  cleavage  to  the 
brief  statements  made  on  a  preceding  page. 

Discoid. — This  is  chiefly  characteristic  of  the  Craniata  but  it 
is  found  occasionally  in  the  Arthropods  (Scorpions).     In  the 


S.C. 


FIG.  116. — Cleavage  in  the  sea-bass,  Serranus  atrarius.  From  H.  V.  Wilson. 
A.  Surface  view  of  blastodisc  in  two-cell  stage.  B.  Vertical  section  through 
four-cell  stage.  C.  Surface  view  of  blastodisc  of  sixteen  cells.  D.  Vertical 
section  through  sixteen-cell  stage.  E.  Vertical  section  through  late  cleavage 
stage,  c.p.,  central  periblast;  m.p.,  marginal  periblast;  s.c.,  segmentation  cavity 
(blastocoel). 

ova  of  these  forms  there  is  often  a  fairly  definite  demarcation 
between  the  protoplasmic  and  deutoplasmic  portions  (Elasmo- 
branchs,  Teleosts,  Reptiles,  Birds)  and  the  cleavage  planes  are 
practically  limited  to  the  former  region,  known  as  the  blasto- 
disc (Figs.  48,  157-159).  The  early  cleavages  may  be  fairly 
regular,  and  approximate  either  radial  or  bilateral  arrange- 
ments, and  as  far  as  the  protoplasm  alone  is  concerned,  the 


CLEAVAGE  241 

cleavage  is  frequently  equal.  When  the  protoplasm  forms 
merely  a  disc  resting  upon  a  large  mass  of  yolk,  it  is  obviously 
impossible  to  speak  of  meridional  or  latitudinal  cleavages; 
hence  the  cleavages  are  described  as  vertical,  either  radial  or 
circular,  and  horizontal.  The  vertical  cleavages  soon  become 
connected  below  the  surface  of  the  ovum  by  horizontal  planes, 
separating  the  lower  surface  of  the  protoplasm  from  the 
underlying  yolk,  and  the  peripheral  circular  cleavages  similarly 
separate  the  protoplasm  from  the  outlying  yolk  (Figs.  116, 
158,  A).  This  is  seen  in  the  Teleosts,  and  in  many  Elasmo- 
branchs,  Reptiles,  and  Birds.  After  the  protoplasmic  blasto- 
disc  is  divided  into  a  number  of  cells,  that  is  after  it  becomes  a 
blastoderm,  other  cleavages  may  occur  parallel  with  the  surface, 
forming  internal  cells  not  visible  upon  the  surface  (Fig.  116), 
and  the  blastoderm  may  thus  come  to  be  many  cells  in  thick- 
ness (Figs.  158,  105,  D). 

Some  interesting  transitions  are  to  be  found  between  total 
unequal  cleavage  and  discoid  cleavage,  in  those  telolecithal 
eggs  where  the  accumulation  of  yolk  is  not  as  great  as  it  is  in 
the  Elasmobranchs,  Teleosts,  and  some  of  the  higher  Craniates. 
Thus  in  the  ganoid,  Amia,  and  some  of  the  Urodeles,  as  well  as 
in  the  squid  (Loligo),  while  cleavage  is  at  first  limited  to  the 
upper  or  animal  pole,  the  earlier  cleavages  gradually  extend 
down  through  the  yolk  mass  and  may  finally  divide  it  into  a 
few  large  cells.  Here  the  more  peripheral  circular  cleavages 
(latitudinal)  do  not  form  any  sharp  separation  between  proto- 
plasm and  deutoplasm,  and  the  yolk  mass  is  for  a  long  time 
only  partially  divided  by  meridional  cleavages  alone  (Fig.  113). 

Superficial. — This  type  of  cleavage  is  characteristic  of  Arthro- 
pods in  general  and  occurs  elsewhere  only  in  a  few  Coelenterates. 
The  central  accumulation  of  the  yolk,  as  it  occurs  in  these  forms, 
is  an  unusual  condition,  and  correlated  with  this  we  find 
several  unusual  features  in  cleavage.  Of  course  in  such  an 
arrangement  the  most  obvious  distinction  between  animal 
and  vegetal  poles  is  entirely  lacking,  and  usually  the  position 
of  the  polar  bodies  and  the  external  form  of  the  egg  are  the 
only  outward  indications  of  the  polarity  of  the  ovum.  Before 


242 


GENERAL  EMBRYOLOGY 


cleavage  begins  the  nuclear  structures  are  located  centrally, 
together  with  a  small  amount  of  protoplasm,  and  surrounding 
this  is  a  dense  mass  of  yolk,  interpenetrated  by  a  very  fine 
network  of  protoplasm.  At  first  nuclear  division  is  not  followed 
by  division  of  the  inert  remainder  of  the  egg  mass.  But  after 
a  varying  number  of  nuclear  divisions,  the  daughter  nuclei 
separate,  each  accompanied  by  a  small  mass  of  protoplasm, 
the  ''protoplasmic  island,"  and  migrate  to  the  surface  of  the 
egg,  continuing  to  multiply  as  they  go  (Figs.  117,  118).  In 


FIG.  117. — Superficial  cleavage  in  the  Decapod,  Dromia.  (Sections.)  After 
Cano.  A,  B.  Intravitelline  divisions  of  the  nucleus.  C,  D,  E.  Formation  of 
yolk-pyramids.  F.  Blastula;  a  superficial  layer  of  cells  enclosing  a  mass  of 
yolk,  n,  nuclei;  p,  yolk  pyramids;  y,  yolk  bodies. 

this  way  the  nuclear  and  cytoplasmic  portions  form  a  kind  of 
superficial  syncytium  leaving  the  condensed  and  undivided 
yolk  centrally. 

Now  either  of  two  things  may  occur.  In  some  forms  (Deca- 
pods, Copepods,  Ostracods,  Amphipods)  cleavages  appear 
almost  simultaneously,  dividing  the  egg  completely  into  a 
number  of  cone-shaped  cells  with  the  apices  directed  centrally; 
these  cells  are  known  as  yolk  pyramids  (Fig.  117).  In  some 
cases  the  formation  of  the  yolk  pyramids  does  not  occur 
simultaneously  throughout  the  egg,  but  occurs  earlier  on  that 
side  of  the  egg  which  corresponds  with  the  ventral  surface  of 


CLEAVAGE 


243 


the  embryo  and  larva.  After  a  variable  but  considerable 
number  of  yolk  pyramids  are  formed  the  planes  of  separation 
gradually  disappear  except  in  the  superficial  protoplasmic 
layer,  which  alone  remains  cellular,  while  the  yolk  again 
becomes  a  solid  mass.  Such  a  process  as  this  is  taken  to  mean 
that  this  type  of  cleavage  may  have  been  derived  from  the 
total  adequal  type. 


FIG.  118. — Cleavage  in  the  beetle,  Hydrophilus.  From  Korschelt  and  Heider, 
after  Heider.  A,  B.  Intravitelline  divisions  of  the  nucleus.  C.  Beginning  of 
the  formation  of  a  superficial  layer  of  cells.  D.  Later  stage  in  formation  of 
"blastoderm,"  or  superficial  cell  layer,  b,  blastoderm,  or  superficial  layer  of 
cells;  d,  yolk;  /,  nuclei  surrounded  by  protoplasm  (protoplasmic  islands) ;  z,  nuclei 
remaining  in  yolk  (merocytes). 

In  other  examples  of  nearly  all  groups  of  Arthropods  the 
yolk  pyramids  are  incompletely  developed  or  even  entirely 
absent  and  the  cleavage  is  strictly  superficial  (Fig.  118). 
Here,  as  in  the  preceding,  blastomeres  may  be  formed  either 
wholly  or  only  partially  around  the  ovum.  The  preliminary 
divisions  of  the  nucleus  and  the  formation  of  protoplasmic 
islands  occur  as  described  above.  In  many  of  the  Insects, 
whose  cleavage  is  typically  superficial,  the  substance  of  the 
ovum  ultimately  becomes  completely  divided  internally. 

The  preceding  classifications  and  descriptions  of  cleavage 
have  been  almost  wholly  morphological  in  their  basis.  But 


244  GENERAL  EMBRYOLOGY 

development  is  a  process,  not  a  succession  of  morphological 
stages,  and  there  remains  to  be  described  the  most  important 
aspect  of  cleavage  as  a  developmental  process.  We  come  then 
to  still  another  classification  of  cleavage  types  as  (1)  deter- 
minate and  (2)  indeterminate. 

Cleavage  is  said  to  be  determinate  when  exact  morphological 
and  physiological  relations  exist  between  the  individual  blasto- 
meres  and,  (a)  specific  structures  in  the  embryo  and  fully 
developed  organism,  and  also  (b)  specific  regions  or  substances 
in  the  ovum.  Each  of  the  products  of  cleavage  is  here  a  true 
organ,  of  particular  and  known  value  in  development:  blasto- 
meres  are  not  interchangeable,  and  their  removal  or  destruction 
may  lead  to  specific,  related  defects  or  abnormalities  in  later 
development  (e.g.,  the  Ascidians).  Cleavage  is  described  as 
indeterminate  when  the  blastomeres  seem  to  have  no  specific 
relation  to  the  structure  of  either  the  egg  or  embryo  and  adult. 
Here  all  the  blastomeres  may  have  equal  value  as  factors  in 
development;  they  are  more  or  less  interchangeable,  and 
removal  or  destruction  leads  only  to  the  loss  of  a  corresponding 
amount  of  substance,  not  to  the  absence  of  any  specific  or 
related  parts  (e.g.,  the  Echinoderms). 

In  order  that  this  classification  should  not  be  misleading,  it 
should  be  said  at  once  that  this  is  a  purely  artificial  distinction, 
based  rather  upon  historical  grounds  than  upon  the  facts  of 
development,  for  these  two  types  are  completely  connected  by 
transitional  conditions,  and  soon  or  late  in  development,  all 
cells  come  to  have  specific,  determined  values. 

Such  a  grouping  as  this,  of  the  varieties  of  cleavage,  obviously 
rests  primarily  upon  a  physiological  rather  than  a  morphological 
basis,  for  cleavage  here  is  regarded  as  a  process  of  development. 
In  all  cases  of  determinate  cleavage  the  essential  fact  is  that 
cleavage  is  not  the  mere  division  of  the  zygote  into  separate 
masses  and  units  which  can  be  moved  about  and  moulded  into 
the  form  of  an  embryo;  cleavage  is  not  merely  a  series  of  cell 
divisions,  not  the  mere  " vegetative  reduplication  of  parts" 
occurring  in  accordance  with  certain  mechanical  rules  like 
those  of  Balfour,  Sachs-Hertwig,  or  Plateau,  mentioned  above. 


CLEAVAGE  245 

We  have  already  seen  that  the  exceptions  to  these  rules  are  so 
numerous  and  so  fundamental  that  they  must  have  some  real 
significance.  It  is  now  clear  that  in  determinate  cleavage  all 
the  details  have  a  significance  that  is  prospective,  looking  toward 
the  structural  and  physiological  characteristics  of  the  larva  or 
fully  formed  organism.  It  has  been  said  regarding  determinate 

cleavage  that  "One  can go  over  every  detail  of 

cleavage,  and  knowing  the  fate  of  the  cells,  can  explain  all  the 
irregularities  and  peculiarities  exhibited"  (Lillie). 

Why  this  should  be  true  is  partly  explained  when  we  remem- 
ber that  the  characters  of  cleavage  and  of  the  fully  developed 
organism  are  both  the  primary  result  of  the  underlying  structure 
of  the  ovum.  Cleavage  stands  as  an  intermediate  process 
between  egg  organization  and  adult  structure;  it  is  one  of  the 
processes  through  which  the  primary  organization  of  the  ovum 
gains  expression  in  adult  form. 

This  view  of  the  cleavage  process  is  by  no  means  the  only, 
or  the  original  view,  but  it  serves  to  bring  out  clearly  the  fact 
that  the  problems  as  to  the  nature  and  causes  of  the  differ- 
entiations occurring  during  the  cleavage  process  are  related  to 
the  problems  of  the  nature  and  causes  of  the  differentiations  of 
adult  structure.  Indeed  these  differentiations  have  a  common 
cause  in  the  structure  and  reactions  of  the  ovum,  and  are  there- 
fore fundamentally  equivalent.  In  our  introductory  chapter 
we  said  that  the  organism  is  specific  at  every  stage,  the  zygote, 
the  group  of  blast omeres,  the  embryo,  the  adult,  are  all  the 
same  specific  organism,  and  the  question  why  the  cleavage 
group  is  what  it  is,  is  the  same  as  the  question  why  the  mature 
organism  has  its  own  specific  and  individual  characteristics. 
The  problems  and  processes  of  development  are  fundamentally 
alike  throughout. 

In  continuing  our  discussion  of  this  determinative  aspect  of 
cleavage  we  shall  make  little  further  attempt  to  distinguish  the 
determinate  and  indeterminate  forms  since  this  separation  is 
clearly  artificial.  The  apparent  differences  between  deter- 
minate and  indeterminate  cleavage  may  arise  from  the  fact 
that  one  of  the  determining  factors  contains  a  variable.  That  is, 


246  GENERAL  EMBRYOLOGY 

the  time  at  which  the  organization  of  the  egg  becomes  suffi- 
ciently complete  to  be  effective  in  determining  the  course  of 
differentiation  of  the  blastomeres,  may  vary.  Thus  cleavage 
would  be  completely  determinate  if  the  egg  organization  were 
entirely  or  largely  completed  before  the  cleavage  process  begins, 
incompletely  determinate  if  the  organization  is  only  partial, 
and  indeterminate  if  the  organization  is  only  slightly 
marked  during  the  earlier  cleavages.  For  egg  organization  is 
progressive,  it  develops.  So  the  determinate  or  indeterminate 
character  of  cleavage  may  depend,  partly  at  least,  upon  the 
relative  time  during  cleavage  at  which  the  organization  becomes 
marked  to  such  an  extent  as  to  determine  the  fate  of  particular 
blastomeres.  Other  factors  obviously  enter  into  the  process 
and  we  shall  review  the  subject  from  another  point  of  view  in 
the  next  chapter. 

We  have  seen  above  that  in  nearly  all  species  the  earliest 
cleavage  planes  are  definitely  related  to  the  polar  axis  of  the 
ovum.  The  polarity  of  the  egg  is  one  of  the  fundamental 
aspects  of  its  organization.  We  have  seen  also  that  the  ovum 
often  contains  formed  substances  of  various  kinds,  both  proto- 
plasmic and  deutoplasmic,  distributed  in  the  cytoplasm  in  a 
definite  and  usually  specific  manner.  It  is  a  common  feature 
of  cleavage  that  the  first  plane  symmetrically  divides  the  egg 
or  that  part  of  it  which  takes  part  in  the  process  of  cleavage. 
And  furthermore,  with  very  few  exceptions,  this  first  cleavage 
plane  coincides  either  precisely  or  approximately  with  the 
median  plane  of  the  embryo  and  adult.  Nereis  is  one  of  the 
few  exceptions  to  this  rule;  in  this  Annelid,  as  in  some  of  the 
Urodeles,  the  second  plane  marks  the  future  median  plane. 

The  factors  which  appear  immediately  to  determine  the  loca- 
tion of  the  first  cleavage  plane  are  mainly  two.  First  and  most 
important  is  the  structure  of  the  ovum  itself,  which  in  many 
cases,  even  in  the  unfertilized  condition,  is  obviously  bilaterally 
symmetrical;  the  first  cleavage  plane  corresponds  closely  with 
this  plane  of  symmetry,  which  is  therefore  determined  by  the 
same  " organizational"  factor  that  determines  the  polarity  and 
other  structural  features  of  the  ovum.  In  other  cases  the 


CLEAVAGE  247 

symmetry  of  the  egg  appears  to  be  radial  or  rotatorial  before 
fertilization  and  is  converted  into  a  bilaterally  symmetrical 
structure  by  the  entrance  of  the  spermatozoon,  the  entrance 
path  of  which  marks  the  plane  of  symmetry  of  the  egg  and 
developing  organism,  and  determines  the  location  of  the  first 
cleavage.  It  is  quite  possible,  though  hardly  demonstrated 
as  yet,  that  even  in  such  cases  there  is  really  an  invisible  bilateral 
structure  of  the  ovum  which  underlies  the  radial  symmetry 
and  really  determines  the  point  at  which  the  sperm  shall  enter. 
In  such  a  case  the  entrance  path  of  the  spermatozoon  would 
itself  be  predetermined  and  could  not  be  regarded  as  a  primary 
factor  in  fixing  the  position  of  the  first  cleavage.  This  would 
obviously  be  the  case  in  many  of  those  eggs  possessing  micro- 
pyles.  But  in  some  eggs  whose  cleavage  is  indeterminate,  even 
though  they  possess  micropyles  (Teleosts),  there  seems  to  be 
no  regularity  in  the  position  of  the  first  cleavage  plane  and  no 
correspondence  between  this  and  any  morphological  char- 
acteristic of  either  ovum  or  adult. 

The  second  cleavage,  usually  at  right  angles  to  the  first, 
ordinarily  corresponds  with  the  median  transverse  axis  of  the 
egg,  embryo,  and  adult.  The  third  cleavage  is  usually  hori- 
zontal and  separates  animal  and  vegetal  poles  and  corresponds 
most  frequently  with  the  separation,  in  the  embryo  and  adult, 
of  the  more  active  animal,  and  less  active  vegetative  tissues. 

The  facts  that  in  all  eggs  of  a  given  species  or  genus  cleavage 
occurs  according  to  a  definite  pattern,  and  that  there  may  be 
an  exact  relation  between  the  individual  blastomeres  of  the 
cleaving  ovum  and  the  tissues  and  organs  of  the  later  organism, 
make  it  possible  to  speak  of  the  "cell  lineage"  (Wilson)  of  an 
organism.  In  forms  with  determinate  cleavage  it  becomes 
possible  to  identify,  even  in  a  comparatively  late  embryonic 
stage,  various  groups  of  cells  as  the  real  lineal  descendants  of 
certain  individual  cells  of  the  earlier  cleavage  group.  In  other 
words,  it  is  in  such  cases  possible  to  trace  the  structures  of  the 
embryo  and  adult  back  to  single  cells  or  parts  of  cells. 

In  order  to  illustrate  the  nature  of  the  facts  of  cell  lineage, 
and  the  completeness  and  exactness  of  the  correspondence  be- 


248  GENERAL  EMBRYOLOGY 

tween  blastomeres  and  differentiated  groups  of  cells  in  the  later 
embryo,  we  may  describe  briefly  a  typical  instance,  using  as  the 
subject  one  of  the  simpler  and  more  regularly  cleaving  types, 
the  Turbellarian,  Planocera,  as  described  by  Surface.  This 
account  of  the  cleavage  of  this  form  should  be  read  with  the 
expectation  of  finding  frequent  " exceptions"  to  the  laws  of 
cleavage  mentioned  above. 

In  Planocera  (Figs.  119,  120)  cleavage  is  total,  unequal,  and 
spiral  (dexiotropic).  The  first  plane  is  meridional  and  divides 
the  egg  into  two  adequal  blastomeres  known  as  AB  and  CD. 
The  division  of  each  of  these  is  also  meridional  but  is  unequal, 
each  forming  a  smaller  and  a  larger  cell,  and  dividing  the  entire 
ovum  into  two  larger,  and  slightly  unequal,  cells  known  as  B 
and  D,  and  two  smaller  cells,  also  slightly  unequal,  known  as 
A  and  C.  Of  these  D  is  the  largest,  and  from  later  develop- 
ment is  known  to  be  posterior  in  position;  B  is  anterior,  A  on  the 
left,  and  C  on  the  right,  as  viewed  from  the  animal  pole  and 
with  reference  to  later  structure. 

The  third  cleavage  is  horizontal,  unequal,  and  strongly 
spiral  (dexiotropic).  As  usual  among  the  Turbellaria  the 
larger  cells  divide  shortly  before  the  smaller.  On  account  of  the 
size  differences  between  the  cells  of  the  upper  and  lower 
quartets,  we  may  describe  the  upper  quartet  of  smaller  cells 
(micromeres)  as  budded  off  from  the  lower  quartet  of  macro- 
meres.  The  quartets  of  micromeres  are  designated  by  small 
letters,  the  macromeres  by  capitals.  Thus  in  the  eight-cell 
stage  we  have  the  first  quartet  of  micromeres,  la,  Ib,  Ic,  Id, 
and  the  first  quartet  of  macromeres,  1A,  IB,  1C,  ID.  Successive 
quartets  are  designated  by  numerical  coefficients,  products  of 
the  division  of  the  quartet  cells  by  exponents.  Thus  in  passing 
from  eight  cells  to  sixteen  the  macromeres  bud  off,  this  time 
in  a  Iseotropic  direction,  a  second  quartet  of  micromeres,  2a,  2b, 
2c,  2d,  the  macromeres  themselves  remaining  known  now  as  2 A, 
2B,  2C,  2D.  Shortly  thereafter  the  first  quartet  of  micromeres 
divide,  also  laeotropically,  forming  two  groups  of  four  cells  each 
known  as  la1,  Ib1,  Ic1,  Id1,  and  la2,  Ib2,  Ic2,  Id2;  the  cells  lying 
toward  the  animal  pole  are  designated  by  the  lower  exponent. 


CLEAVAGE  249 

The  fifth  cleavage,  dividing  the  sixteen  cells  into  thirty-two, 
in  general  resembles  the  preceding  but  is  dexiotropic  through- 
out. As  the  result  w£  have  the  macromeres,  3 A,  3B,  3C,  3D; 
a  third  quartet  of  micromeres,  3a,  3b,  3c,  3d;  the  second  quartet 
of  micromeres  divides  into  2a*,  2b\  2c*,  2d*,  and  2a2,  2b2,  2c2, 
2d2,  while  the  eight  cells  previously  derived  from  the  first 
quartet  of  micromeres  now  form  la11,  la12,  la21,  la22,  Ib11,  lb12, 
lb21,  lb22,  lc11,  lc12,  lc21,  lc22,  Id11,  Id12,  Id21,  Id22. 

After  thirty-two  cells  have  been  formed  in  this  fairly  regular 
fashion,  the  rhythm  of  cleavage  becomes  modified  so  that  there 
follow  stages  of  40,  44,  45,  53,  61  cells,  etc. 

It  is  unnecessary  for  us  to  go  farther  with  the  details  of 
these  cleavages  save  in  one  particular.  After  the  thirty-two- 
cell  stage  the  macromeres  divide  again  unequally  giving  off  a 
group  of  large  cells  which  contain  most  of  the  deutoplasm  of 
the  original  ovum.  In  spite  of  their  size  relations  these  large 
cells  are  known  as  the  fourth  quartet  of  micromeres,  4a,  4b,  4c, 
4d,  and  the  remaining  smaller  cells  as  the  fourth  quartet  of 
macromeres,  4A,  4B,  4C,  4D.  This  contradiction  in  termin- 
ology is  justified  by  the  later  history  of  these  cells. 

We  may  now  consider  the  fates  of  these  various  groups  of 
blastomeres.  Quoting  from  Surface,  "From  the  first  quartet 
[of  micromeres]  arises  the  ectoderm,  covering  the  anterior 
and  dorsal  portions  of  the  body.  From  cells  of  this  quartet  four 
strings  of  cells  bud  into  the  interior  of  the  embryo  and  form  the 
ganglion.  The  eyes  arise  in  ectodermal  cells  of  this  quartet. 
The  second  quartet  gives  rise  to  the  larger  portion  of  the  ecto- 
derm on  the  ventral  and  posterior  regions  of  the  body.  From 
cells  of  this  quartet  is  formed  most  of  the  ectodermal  pharynx. 
A  portion  of  the  second  quartet  is  budded  into  the  embryo  and 
forms  mesoderm.  From  this  source  arises  probably  only  that 
mesoderm  formed  around  the  blastopore  and  which  is  later 
concerned  in  the  structures  of  the  pharynx. 

"The  third  quartet  consists  of  small  cells  from  which  appar- 
ently only  ectoderm  is  derived.  The  individual  divisions  of 
these  cells  have  not  been  traced  very  far,  but  there  is  every 
reason  to  believe  that  they  form  ectoderm  only. 


250 


GENERAL  EMBRYOLOGY 


FIG.  119. — Cleavage  and  cell  lineage  in  the  Polyclad  Turbellarian,  Planocera 
inquilina.  From  Surface.  A.  Egg  during  the  first  cleavage;  side  view.  The 
cell  C-D  is  slightly  larger  than  A-B.  B.  Four-cell  stage,  from  animal  pole.  C. 
Formation  of  first  quartet,  from  right  side,  showing  spiral  cleavage  (dexiotropic). 
D.  Eight-cell  stage,  from  animal  pole.  E.  Eight-cells  dividing  into  sixteen, 
showing  laeotropic  division.  The  division  of  the  cells  of  the  D  quadrant  is  in 
advance  of  the  others.  F.  Sixteen-cells,  from  animal  pole.  G.  Sixteen-cells, 
from  vegetal  pole. 


CLEAVAGE 


251 


FIG.  120. — Continuation  of  Fig.  119.  H.  Dexiotropic  division  of  lal-\d>  and 
of  2a-2d.  From  animal  pole.  I.  Thirty-two-cells,  from  animal  pole.  J. 
Thirty-two-cells  from  vegetal  pole.  K.  Thirty-two-cells  from  right  side.  L. 
Late  cleavage  showing  the  history  of  cells  4a-4d  and  4A -4D.  M .  Optical  section 
of  a  much  later  stage,  viewed  from  near  the  vegetqj  pole.  The  mesoderm  bands 
are  stippled. 


252 


GENERAL  EMBRYOLOGY 


"The  history  of  the  fourth  quartet  is  peculiar 

The  posterior  cell  4d  is  the  mesentoblast,  from  which  the  ali- 
mentary canal  and  a  portion  of  the  mesoderm  arise.  The 
other  three  cells  of  the  fourth  quartet,  4a,  4b,  4c,  do  not  divide 
as  long  as  their  history  can  be  traced.  They,  however,  break 
up  into  a  large  number  of  homogeneous  yolk  spheres  which  are 
absorbed  by  the  endoderm  cells.  The  large  nuclei  of  these 
three  cells  can  be  traced  until  the  alimentary  canal  is  partly 
formed. 

TABLE  OF  THE  CELL  LINEAGE  OF  PLANOCERA 

Condensed  from  Surface. 


A  (left) 


Zygotc 


AB 


B  (ant.) 


CD 


C  (rt.) 


D  (post.) 


la 

-  i  u« 

I  "•  {  ;::: 

1A 

f  2a       S  2al 

if 

2A     /  3a 
{  3A 

Ib 

r  ib1 
1  Ib2 

Ib11 
Ib12 
Ib21 
Ib22 

IB 

f  2b 

[  2B 

2b1 
2b2 
3b 
3B 

(  i0i     J   1(;11 
\  lc12 

lc 

\             \  1p21 

1      1  P2        J      1C 

\  lc22 

(  2C1 

1C 

J2c      , 
[  2C     . 

[  2c2 
3c 
3C 

Id 

jid-  , 

[  Id2    . 

Id11 
Id12 
Id21 
Id22 

f2d      ^ 

2dl 
2d2 

ID 

\            (  3d 

CLEAVAGE 


253 


la111  ] 
Ib111  ( 
Ic111 
Id111 


4a,  4A 

4b,  4B 

4c,  4C 

4D 


apical  cells. 


Ja112212 
1^112212 


primary  ganglion 


2a 
2b 

2d 


mesoblast. 


yolk  (no  cell 
descendants) . 


4d 


4d* — alimentary  canal  (endoderm). 

I"  4d211 — probably  endo- 
derm. 
4d212 — mesoderm  (right 

"mesoblast  band"). 
4d221 — probably   endo- 
derm. 

4<p22 — mesoderm    (left 
"mesoblast  band"). 


4dJ 


4d21 


4d22 


The  remaining  cells  form  covering  ectoderm. 
Ectoderm  of  first  quartet — anterior  and  dorsal,  including  eyes. 
Ectoderm  of  second  quartet — posterior  and  ventral,  including  pharyngeal. 
Mesoderm  of  second  quartet — blastoporal  (pharyngeal). 

"The  nuclei  of  the  small  macromeres  [4A,  4B,  4C,  4D]  show 
evidences  of  degeneration.  These  do  not  divide  as  long  as 

they  can  be  followed and  it  seems  probable  that 

they  degenerate  without  giving  rise  to  any  morphological 
structure." 

This  cell  lineage  of  Planocera  is  summarized  incompletely  in 
the  accompanying  table. 

It  is  interesting  to  compare  with  this  lineage  of  Planocera 
that  of  A  scarisj  described  by  Zur  Strassen,  which  is  somewhat 
less  regular.  This  is  particularly  interesting  as  it  shows 
clearly  the  history  of  the  germ  cells,  which  become  wholly 
separate  from  somatic  cells  in  the  sixteen-cell  stage.  Cleavage 
of  Ascaris  (Fig.  121)  is  bilateral  but  more  or  less  irregular, 
particularly  in  its  rhythms,  so  that  without  attempting  to 
apply  the  ordinary  terminology  completely  we  may  summarize 
the  early  cell  history  in  the  table  accompanying  (p.  255). 

The  cell  lineage  of  a  considerable  number  of  organisms  has 
been  definitely  traced,  often  in  much  greater  detail  than  we 
have  indicated.  The  histories  best  known  are  found  among 
the  Platyhelminth.es,  Nemathelminthes,  Nemertinea,  Annulata, 
Trochelminthes,  Mollusca,  and  Tunicata.  Of  course  the  eggs  of 
many  classes  and  phyla  show  no  such  regularity,  for  as  we  have 


254 


GENERAL  EMBRYOLOGY 


pointed  out,  cleavage  may  be  irregular  as  well  as  indeterminate. 
And  in  many  of  the  groups  named  above,  normal  development 


FIG.  121. — Cleavage  in  Ascaris  megalocephala  bivalens.  From  Jenkinson,  after 
Boveri.  1.  Division  of  the  two-cell  stage.  Elimination  of  chromatin  in  the 
somatic  cell  Si(AB).  la.  Chromosomes  of  the  cell  S\(AB).  2.  Four-cell  stage 
(T-form).  In  A  and  B  can  be  seen  the  eliminated  chromatin.  The  cell  Pi  has 
divided  into  a  somatic  cell  Sz(EMSt),  in  the  descendants  of  which  chromatin 
elimination  occurs,  and  the  cell  Pa.  3.  Four-cell  stage  (lozenge-form).  A  is 
anterior,  A  and  B,  dorsal.  4.  Continued  chromatin  elimination  in  somatic 
cells.  p2  has  divided  into  Ps,  and  Ss(C) — secondary  ectoderm,  a,  b,  primary 
ectoderm  of  right  side,  «,  0,  of  left  side.  5.  The  endoderm  cell  has  been  formed 
and  has  divided  (Ei,  Ez).  Ps  has  divided  into  P-i,  the  primordial  germ  cell,  and 
S4(d),  tertiary  ectoderm.  6.  Ventral  view  at  the  beginning  of  invagination. 
Elimination  of  chromatin  in  S«(D).  The  four  endoderm  cells  (E)  beginning  to 
invaginate.  On  each  side  two  mesoderm  cells  (M)  in  which  granular  chromo- 
somes may  be  seen,  and  two  stomodaeal  cells  (St). 

may  occur  even  though  interrupted  by  the  removal  of  parts, 
by  pressure,  etc. 


CLEAVAGE  255 

TABLE  OF  THE  CELL  LINEAGE  OF  ASCARIS 

Modified  from  Zur  Strassen 
(Letters  in  parenthesis  are  the  notation  of  Zur  Strassen,  Boveri  and  others) 


a1 

(al) 

a 

a2 

r  A                  (a) 

(all) 

(A) 

a1 

(al) 

a 

(a) 

a2 
(all) 

AB     - 

b1 

(SJ 

(bl) 

b 

b2 

B                  (b) 

(bll) 

(B) 

6' 

1   b 

OH) 

(  (/?) 

62 

1 

bfi   < 

r  c 

(MSt) 
(EMSt) 
I   (E) 

r  %  {  cc;2 

(mst) 
c2(left)    I    c22 

(/"*)        }  Ccll 
f  #  W         L 
(ED        r  L, 

c2(left)    \     22 

(EH)             dn   > 

mesoblast  I  (posterior), 
stomodgeal  cells  (anterior), 
mesoblast  I  (posterior), 
stomodaeal  cells  (anterior), 
(rt.)     ] 

(left)     I                      T 
,.*      >  endoderm  I. 
(rt.) 

(left)    J 

d1 

>  ectoderm  II. 

CD 

M            \  d      J 

(Pi) 

r  d 

(V           /   d21  ~ 

>   mesoderm  III. 

(S,) 

/  \           1   d22  , 

D 

*         -»  1 

-  mesoderm  II. 

J-'              > 

(P3) 

s>   £ 
?>   ^ 

*.  ^  *}           rn  ^ 

f  d211  (left)  I 
(ant.)     {                          primordial 
1    d212  (rt.) 

,  ,  /  d22'  (left)       ^ 
(post.)                             cells. 

*  Primordial  germ  cell. 

The  study  of  cleavage  from  this  point  of  view  discloses  the 
fact,  of  the  utmost  importance  in  development,  that  blasto- 
meres  may  be  individually  and  specifically  recognizable  as 
morphological  and  morphogenetic  units.  They  bear  much  the 
same  relation  to  the  whole  cell  group  that  the  organs  and  tissues 


256 


GENERAL  EMBRYOLOGY 


bear  to  the  embryo  or  later  organism.  As  distinct  morpho- 
logical and  physiological  units  they  represent  real  differentia- 
tions at  a  very  early  stage  of  development,  and  may  truly  be 
said  to  form  embryonic  rudiments  of  structures  appearing  later 
in  the  form  of  germ  layers  or  derivatives  of  these. 


FIG.  122. — Diagrams  illustrating  the  value  of  the  quartets  in  three  animals. 
From  Wilson,  " Cell."  Ectoplasm  is  unshaded;  mesoplasm  is  dotted;  endoplasm 
is  vertically  ruled.  A.  The  Polyclad,  Leptoplana,  showing  mesoplasm  formation 
in  second  quartet.  (Compare  Planocera,  Fig.  120,  where  Surface  finds  meso- 
plasm in  cell  4d  descendants.)  B.  The  Gasteropod,  Crepidula.  C.  The 
Pelecypod,  Unio. 

Not  only  this  but  comparison  of  the  cell  lineages  of  different 
classes  and  phyla  often  brings  out  the  fact  that  particular 
cells  can  be  identified  and  compared  in  diverse  groups  of  ani- 
mals, making  it  possible  to  apply  the  idea  of  homology  to  blas- 
tomeres  and  groups  of  blastomeres  in  the  early  embryo,  as  well 


CLEAVAGE  257 

as  to  the  organs  and  parts  of  the  fully  formed  organism.  Cells 
may  be  vestigial,  rudimentary,  and  the  like,  in  the  same  way 
that  organs  may  be.  The  three  or  four  successively  formed 
quartets  of  micromeres  or  even  an  individual  cell,  for  example 
that  known  as  4d,  can  be  identified  and  homologized  both  in 
origin  and  in  fate,  in  the  phyla  Platyhelminthes,  Annulata, 
and  Mollusca  (Fig.  122).  Wilson  has  written  ("The  Cell," 
etc.,  page  416):  "Thus  we  find  that  the  cleavage  of  polyclades, 
annelids  and  gasteropods  shows  a  really  wonderful  argeement 
in  form,  yet  the  individual  cells  differ  markedly  in  prospective 
value.  In  all  of  these  forms  three  quartets  of  micromeres  are 
successively  formed  according  to  exactly  the  same  remarkable 
law  of  alternation  of  the  spirals;  and,  in  all,  the  posterior  cell 
of  a  fourth  quartet  lies  at  the  hinder  end  of  the  embryo  in 
precisely  the  same  geometrical  relation  to  the  remainder  of  the 
embryo;  yet  in  the  gasteropods  and  annelids  this  cell  gives  rise 
to  the  mesoblast-bands  and  their  products,  in  the  polyclade  to  a 
part  of  the  archenteron,  while  important  differences  also  exist 
in  the  value  of  the  other  quartets."  (It  should  be  added  that 
in  the  Polyclad,  Planocera,  the  particular  cell  mentioned  gives 
rise  to  mesoblast  also.) 

Such  conditions  also  illustrate  how  the  facts  of  embryology 
may  have  a  certain  value,  often  very  great,  as  evidence  upon 
phylogenetic  problems. 

Often  these  similarities  of  structure  can  be  carried  back  into 
the  pre-cleavage  stage,  and  in  the  uncleaved  zygote  or  ovum 
before  fertilization,  substances  can  be  identified  which  later 
become  contained  within  restricted  groups  of  similar  cells. 
So  that  cleavage  is  in  part  to  be  regarded  as  a  process  by  which 
specific  substances  or  regions  of  the  egg  become  segregated  in 
different  regions  of  the  embryo,  where  each  continues  its 
normal  differentiation  during  later  developmental  stages  and 
gives  rise  to  specific  tissues  or  organs.  In  other  words  the 
process  of  differentiation  is  not  limited  to  the  later  stages  of 
development  following  cleavage;  it  occurs  during  and  even 
preceding  cleavage.  In  Ascaris,  for  example,*  each  of  the  two 
cells  resulting  from  the  first  cleavage  is  specific;  in  other 


258  GENERAL  EMBRYOLOGY 

forms  each  of  the  four  or  eight  cells  has  already  become  differ- 
entiated. In  forms  like  the  Ascidians  (Conklin)  true  differ- 
entiation has  commenced  in  the  uncleaved  ova  and  has  been 
carried  to  a  very  pronounced  degree.  And  it  is  not  at  all 
unlikely  that  these  differentiations  of  the  undivided  egg  may 
represent  the  most  essential  and  most  fundamental  differen- 
tiations of  the  organsim.  This  aspect  of  cleavage  cannot  be 
discussed  satisfactorily  here  without  encroaching  widely  upon 
the  subject  of  the  next  chapter  and  it  is  therefore  left  at  this 
point. 

It  is  to  be  noted,  however,  in  conclusion  that  while  cleavage 
may  have  an  important  chemical  significance  of  general  char- 
acter, and  a  general  physiological  significance,  yet  the  process 
is  primarily  a  specific  process  of  development  and  not  mere 
cell  multiplication.  The  process  is  closely  related  in  all  its 
details  to  the  structure  of  the  egg  and  also  to  the  structure  of 
the  adult,  since  this  too  is  similarly  related  to  egg  structure. 
Many  of  the  details  of  cleavage  do  not  occur  according  to  phys- 
ical laws  based  upon  space  and  time  relations  of  the  parts, 
but  departures  from  what  we  should  expect  on  the  basis  of 
such  laws  may  result,  (a)  from  the  historical  factor  of  the 
relationships  of  organisms  and  the  process  of  descent,  (6)  from 
the  teleological  factor,  for  cleavage  has  a  prospective  signifi- 
cance, looking  forward,  as  well  as  a  retrospective  significance — 
cleavage  has  its  promorphology  as  well  as  its  morphology. 

REFERENCES  TO  LITERATURE 

BALFOUR,  F.  M.,  (See  ref.  Ch.  III'.) 

BEARD,  J.,  (See  ref.  Ch.  III.) 

BOVERI,  T.,  Die  Polaritat  von  Ovocyte,  Ei  und  Larve  des  Strongylocen- 
trotus  lividus.  Zool.  Jahrb.  14.  1901. 

BRESSLAU,  E.,  Beitrage  zur  Entwicklungsgeschichte  der  Turbellarien. 
I.  Die  Entwicklung  der  Rhabdocolen  und  Alloicolen.  Zeit.  wiss. 
Zool.  76.  1904. 

CONKLIN,  E.  G.,  (See  ref.  Ch.  II,  III.) 

DRIESCH,  H.,  Entwicklungsmechanische  Studien.  VIII.  Ueber  Varia- 
tion der  Mikromerenbildung.  (Wirkung  von  Verdiinnung  des 
Meerwasser).  Mitt.  Stat.  Neapel.  11.  1893. 


CLEAVAGE  259 

GODLEWSKI,  E.,  Plasma  und  Kernsubstanz  in  der  normalen  und  der 

durch  aussere  Factoren  veranderten  Entwicklung  der  Echiniden. 

Arch.  Entw.-Mech.     26.     1908. 
HARGITT,   C.  W.,   Some  Problems  of  Coelenterate    Ontogeny.     Jour. 

Morph.     22.     1911. 
HERTWIG,  O.,  Das  Problem  der  Befruchtung  und  der  Isotropie  des  Eies, 

eine  Theorie  der  Vererbung.     Jena.  Zeit.     18  (11).     1885. 
KORSCHELT  UXD  HEiDER,  Lehrbuch,  etc.     Ill  Abschnitt.     Furchung 

und  Keimblatterbildung.     Jena.     1909. 

LILLIE,  F.  R.,  Adaptation  in  Cleavage.     Woods  Holl  Biol.  Lect.     1899. 
MASIXG,  E.,  (See  ref.  Ch.  V.) 
MOEXKHAUS,  W.  J.,  (See  ref.  Ch.  II.) 
ROBERT,  A.,  Recherches   sur  le  developpement  des  Troques.     Arch. 

Zool.     Exp.  (III).     10.     1903. 

RUCKERT,  J.,  Ueber  das  Selbstandigbleiben  der  vaterlichen  und  mutter- 
lichen  Kernsubstanz  wahrend  der  ersten  Entwicklung  des  befruch- 

teten  Cyclops-Eies.     Arch.  mikr.  Anat.     45.     1895. 
SACHS,  J.,  Ueber  die  Anordnung  der  Zellen  in  jiingsten  Pflanzentheilen. 

Arbeiten  Bot.  Inst.     Wtirzburg.     2.     1882. 
SELEXKA,  E.,  Die  Keimblatter  der  Echinodermen.     Stud,  uber  Entw. 

II.     Wiesbaden.     1883. 
SURFACE,  F.   M.,  The  Early  Development  of  a  Polyclad,   Planocera 

inquilina,  Wh.     Proc.  Acad.  Nat.  Sci.  Philadelphia.     1907. 
WATASE",   S.,   Studies   on   Cephalopods.     I.     Cleavage   of  the   Ovum. 

Jour.  Morph.     4.     1891. 
WILSOX,  E.  B.,  The  Cell-Lineage  of  Nereis.     Jour.  Morph.     6.     1892. 

Cell    Lineage   and    Ancestral    Reminiscence.     Woods    Holl    Biol. 

Lect.  1899.     (See  also  ref.  Ch.  II.) 
WILSOX,  H.  V.,  The  Embryology  of  the  Sea-Bass.     (Serranus  atrarius.) 

Bull.  U.  S.  Fish  Com.     9.     1889.     (1891). 
ZELEXY,  C.,  Experiments  on  the  Localization  of  Developmental  Factors 

in  the  Nemertine  Egg.     Jour.  Exp.  Zool.     1.     1904. 
ZIEGLER,  H.  E.,  Experimentelle  Studien  tiber  Zelltheilung.     III.     Die 

Furchungszellen  von  Beroe  ovata.     Arch.  Entw.-Mech.     7.     1898. 
ZUR  STRASSEX,  0.,  Embryonalentwickelung  der  Ascaris  megalocephala. 

Arch.  Entw.-Mech.     3.     1896.     Ueber  die  Lage  der  Centrosomen 

in  ruhenden  Zellen.     Arch.  Entw.-Mech.     12.     1901. 


CHAPTER  VII 

THE  GERM  CELLS  AND  THE  PROCESSES  OF 

DIFFERENTIATION,  HEREDITY,  AND 

SEX  DETERMINATION 

THE  problem  of  heredity  is  the  problem  of  development. 
The  student  of  heredity  is  concerned  primarily  with  the  com- 
parison of  the  traits  of  adult  organisms  as  they  appear  in  suc- 
cessive generations,  and  with  the  methods  of  the  distribution 
of  distinctively  individual  parental  characteristics  among 
successive  generations  of  offspring.  The  student  of  develop- 
ment is  concerned  primarily  with  the  genesis  of  the  traits  of 
the  individual,  with  that  continuous  and  orderly  sequence  of 
changes  that  gives  to  the  single-celled  zygote  the  final  form  of 
the  fully  matured  animal. 

It  might  seem,  therefore,  that  any  consideration  of  the 
problems  of  heredity  is  somewhat  out  of  place  in  an  account 
of  the  processes  of  development.  At  one  time  this  might 
justly  have  been  urged.  But  to-day  the  students  of  heredity 
and  of  embryology  have  in  common  much  that  is  fundamental. 
Their  interests  meet  in  the  ideas  that  the  organism  is  a  specific 
creature  at  every  stage  of  its  existence,  from  zygote  to  adult; 
that  the  qualities  of  each  later  stage  are  conditioned  by  those 
of  an  earlier;  and  so  ultimately  the  structural  and  functional 
differentiations  of  the  adult  must  be  traced  back  to  correspond- 
ing differentiations  of  the  zygote,  or  even  to  pre-conjuga- 
tion  phases  of  the  gametes.  It  is  the  common  endeavor  of 
the  students  of  embryology  and  of  genetics  to  answer  the 
question  why  the  egg  of  a  star-fish  develops  into  a  star-fish, 
with  the  characteristics  of  its  parents,  rather  than  into  a  sea- 
urchin,  although  it  may  develop  in  the  same  dish  of  water 
with  other  eggs  that  do  develop  into  sea-urchins. 

260 


DIFFERENTIATION,  HEREDITY,  SEX  261 

Heredity  is  the  fact  of  resemblance  between  offspring  and 
parents,  not  the  resemblance  of  adult  stages  alone,  but  the 
likeness  at  all  corresponding  ages.  That  the  individual  ova  of 
Asterias  vulgaris  are  alike,  that  the  cleavage  processes,  blastulas, 
gastrulas,  larvae,  and  adolescent  stages  of  all  the  individuals 
of  this  species  are  essentially  alike,  in  structure  and  in  behavior 
—all  this  is  similarly  the  fact  of  heredity.  A  specific  kind 
of  protoplasm  is  never,  whatever  its  form,  anything  other  than 
that  specific  kind. 

In  other  words,  the  interest  of  the  embryologist  in  the  prob- 
lems as  to  why  the  ovum  develops  as  it  does,  passes  from  one 
condition  to  another  as  it  does,  and  finally  produces  the  kind 
of  adult  that  it  does,  is  essentially  an  interest  in  the  problem 
of  heredity — the  problem  of  organismal  specificity.  This  is 
the  central  point  of  embryological  study.  Consequently  we 
are  fully  justified  in  considering  in  this  place,  these  general 
problems  of  the  relation  of  the  facts  of  cytology  and  embryology 
to  the  facts  of  parental  and  specific  likeness  of  organisms. 
Failure  to  do  so  would  mean  the  omission  of  the  most  vital 
topic  around  which  much,  perhaps  it  would  not  be  going  too 
far  to  say  most,  of  recent  embryological  investigation  has 
centered,  and  upon  which  it  is  to-day  focussed. 

The  answers  which  the  facts  of  embryology  have  to  offer  to 
these  fundamental  questions  are  still  rather  vague  and  uncer- 
tain. Most  of  them  are  stated  in  the  potential  mood  and  must 
still  be  framed  as  hypotheses.  But  although  the  facts  here  may 
be  much  clearer  than  their  significance,  we  must  attempt  a  state- 
ment of  both ;  and  it  goes  almost  without  saying  that  our  treat- 
ment of  both  must  be  as  brief  and  as  elementary  as  possible; 
this  is  not  the  place  for  extended  consideration  of  hypothetical 
views,  however  great  the  importance  of  the  central  ideas. 

"We  may  address  ourselves,  therefore,  to  a  survey  and  brief 
analysis  of  the  answers  which  have  been  given  to  the  question 
why  the  organism  develops  in  the  way  it  does.  First,  let  us 
recall  the  idea,  stated  briefly  in  the  introductory  chapter,  that 
development  is  a  form  of  behavior — a  series  of  reactions.  In 
any  organic  reaction  the  two  factors  of  external  and  internal 


262  GENERAL  EMBRYOLOGY 

conditions  are  involved.  The  reactions  of  the  ovum  in  cleaving, 
of  the  blastula  in  gastrulating,  and  the  like,  are  of  a  general 
nature,  i.e.,  the  external  conditions  involved  may  be  consider- 
ably varied,  within  limits,  and  yet  produce  the  same  response 
on  the  part  of  the  organism.  Emphasis  thus  is  placed  upon  the 
internal  factors  in  the  reactions  of  development,  for  on  account 
of  the  definite  character  of  habits  of  life  and  of  spawning,  the 
normal  external  conditions  of  development  are  sufficiently 
uniform  to  produce  a  series  of  reactions,  a  development,  which 
is  also  uniform,  i.e.,  normal.  Of  course  it  is  easily  possible  to 
alter,  artificially,  the  external  as  well  as  the  internal  conditions 
of  development  and  the  results  of  such  alteration  often  lead, 
as  we  shall  see,  to  important  ideas  regarding  normal  or  char- 
acteristic embryonic  behavior. 

Development  is,  then,  a  series  of  reactions,  one  condition 
leading  to  the  next;  and  the  primary  factor  in  determining  the 
quality  of  each  reaction  is  the  internal  condition  or  structure, 
both  morphological  and  physiological,  of  the  organism,  whether 
it  be  ovum,  zygote,  blastula,  larva,  or  adolescent  individual. 

Throughout  the  efforts  to  solve  the  problem  of  individual 
development,  the  attempt  has  always  been  to  explain  existing 
differentiations  as  being  dependent  upon  some  preexisting 
differentiation,  related  but  of  a  different  kind.  Thus  at  one 
time  the  earliest  differentiations  that  became  visible  in  the 
developing  organism  were  the  germ  layers,  and  these  were 
consequently  regarded  as  the  fundamental  differentiations 
of  the  embryo,  determining  its  subsequent  history.  Next, 
differentiations  among  the  cleavage  cells  were  noted  and 
emphasized  as  primary.  Then  as  technique  improved,  and 
the  subject  of  cytology  developed,  attention  became  focussed 
upon  the  nucleus  and  its  organs  not  only  as  the  centers  of  cell 
life,  but  as  the  structures  primarily  concerned  in  the  differen- 
tiations of  the  developing  egg  cell.  And  lately,  chiefly  as  a 
result  of  experimental  analysis  of  the  processes  of  development, 
rather  than  as  the  consequence  of  observation  alone,  structural 
differentiations  of  the  cytoplasmic  portion  of  the  ovum  have 
occupied  the  center  of  interest  as  determining  factors  in 


DIFFERENTIATION,  HEREDITY,  SEX  263 

development.  This  succession  of  views  represents  rather  the 
order  of  their  general  acceptance  as  working  bases,  than  of 
their  discovery  and  individual  promulgation. 

In  accordance  with  this  historical  succession  of  ideas  regarding 
the  nature  of  the  underlying  differentiations  in  development, 
we  may  outline  briefly  three  general  hypotheses  of  the  causes  of 
differentiation.  We  may  omit,  as  being  now  of  historical 
interest  chiefly,  any  further  reference  to  the  germ  layers  as  the 
primary  determiners  of  development,  and  may  begin  with  the 
idea  of  Pfliiger,  that  the  egg  and  the  blastomere  group  are 
homogeneous  or  isotropic  throughout,  and  that  the  early 
developmental  processes  of  cleavage  are  nothing  more  than 
indifferent  multiplications  of  similar  units,  resulting  in  the 
formation  of  blocks  out  of  which  later  differentiating  structures 
may  be  built.  During  the  early  '90's  this  view  was  quite 
prevalent,  and  especially  favored  by  such  embryologists  as 
Oscar  Hertwig  and  Driesch,  who  developed  it  somewhat  farther 
into  what  has  been  termed  the  "cell  interaction"  hypothesis. 
According  to  this  hypothesis,  while  the  cells  of  the  blastomere 
group  are  essentially  similar  and  equivalent  in  their  poten- 
tialities (" prospective  potency,"  Driesch),  differentiation  exists 
among  them  by  virtue  of  their  relative  position  in  the  cell 
group — not  through  any  actual,  individual,  intracellular 
differentiation.  Stated  in  the  words  of  Wilson  (The  Cell,  etc., 
page  415),  two  sentences  of  Driesch  summarize  this  view  as 
follows :  "  The  blast omeres  of  the  sea-urchin  are  to  be  regarded 
as  forming  a  uniform  material,  and  they  may  be  thrown  about, 
like  balls  in  a  pile,  without  in  the  least  degree  impairing  thereby 
the  normal  power  of  development."  "The  relative  position 
of  a  blastomere  in  the  whole  determines  in  general  what  develops 
from  it;  if  its  position  be  changed,  it  gives  rise  to  something 
different;  in  other  words,  its  prospective  value  ["prospective 
significance,"  Driesch]  is  a  function  of  its  position." 

In  itself,  then,  the  cell  interaction  hypothesis  offers  no 
explanation  of  differentiation  or  development,  for  it  throws 
back  upon  some  unknown  factor  the  real  cause  of  differentiation 
through  position.  Later  investigation,  chiefly  experimental,  has 


264  GENERAL  EMBRYOLOGY 

shown  clearly  that  cell  interactions  alone  play  but  a  small 
part  indeed  in  the  process  of  differentiation,  and  has  led  to  the 
search  for  that  underlying  factor  or  group  of  factors.  And 
while  Driesch  himself  concludes  that  no  explanation  is  possible 
in  known  terms  of  matter  or  energy,  and  relies  upon  an  unknown, 
and  therefore  metaphysical,  factor,  the  great  majority  of  em- 
bryologists  believe  that  the  question  is  still  susceptible  of 
further  scientific  analysis.  We  find  two  general  hypotheses 
regarding  the  nature  of  the  causes  or  conditions  of  differentia- 
tion, and  since  differentiations  are  always  specific  we  may  speak 
of  these  as  also  hypotheses  as  to  the  causes  of  those  resemblances 
among  generations  of  organisms  which  we  call  in  a  word, 
heredity. 

The  first  of  these  is  the  hypothesis  of  "  germinal  localization  " 
or  " germinal,  organ-forming  regions"  associated  primarily 
with  the  names  of  His,  Lankester,  and  Whitman.  The  essen- 
tials of  this  hypothesis  in  its  present  form  may  be  stated  as 
follows.  The  cytoplasm  of  the  ovum  before  development  (i.e., 
cleavage)  begins,  has  a  definite  structure  or  morphology  of  its 
own,  such  that  particular  regions  or  substances,  by  effecting 
specific  developmental  reactions,  are  seen  to  correspond  with, 
or  to  lead  to  the  formation  of,  particular  tissues  or  structures 
of  later  stages  and  of  the  fully  developed  organism.  The 
cytoplasm  is  conceived  as  a  mosaic-work  of  physiological  units 
which  have  not  only  a  definite  morphology  but  a  definite  pro- 
morphology,  looking  toward  the  structure  of  the  mature  individ- 
ual. This  immediately  suggests  the  old  idea  of  "preforma- 
tion"  but  it  omits,  of  course,  the  naive  crudities  of  this  con- 
ception and  rests  upon  the  idea  that  certain  structures  and 
regions  of  the  egg  cytoplasm  are  in  direct  genetic  relation  with 
corresponding  structures  and  regions  differentiating  later  by  a 
true  process  of  development  or  epigenesis.  These  germinal 
structures  have  specific  reference,  but  not  resemblance,  to  the 
parts  of  the  mature  organism.  This  predetermination  may  be 
only  general  to  begin  with,  but  it  becomes  more  complete  and 
specific  as  one  condition  succeeds  another,  the  cytoplasmic 
structure  of  the  ovum  representing  in  the  beginning  all  there 


DIFFERENTIATION,  HEREDITY,  SEX  265 

is  of  definite,  specific,  organismal  structure.  This  cytoplasmic 
structure  of  the  germ  is  regarded  as  continuous  from  one 
generation  of  ova  to  the  next,  through  the  germ  within  the 
organism,  and  it  thus  serves  as  the  physical  basis  of  heredity. 

As  more  or  less  opposed  to  this  conception  we  have  a  group 
of  hypotheses  which  may  collectively  be  termed  the  hypothesis 
of  "  nuclear  analysis."  Here  the  nucleus  alone  is  regarded  as 
that  part  of  the  germ  or  zygote  which  bears  a  specific  relation  to 
later  differentiations,  and  within  the  nucleus,  the  chromosomes 
are  the  elements  chiefly  concerned.  Chromosomes  are  supposed 
to  possess  a  structural  predetermination  that  is  promorpho- 
logical;  they  are  unlike  and  individually  behave  specifically 
in  determining  the  characteristics  of  developmental  reactions. 
This  hypothesis,  in  its  various  forms,  is  associated  chiefly  with 
the  names  of  Nageli,  Roux,  Weismann,  DeVries,  and  Oscar 
Hertwig.  It  will  be  recognized  as  also  preformational  in  its 
essentials;  specific  configurations  of  chromatin  represent 
potentially,  corresponding  embryonic  and  adult  traits,  which 
become  actual  by  a  truly  epigenetic  series  of  developmental 
reactions. 

It  has  been  pointed  out  frequently  that  this  hypothesis  really 
transfers  the  idea  of  germinal  localization  from  the  cytoplasm 
to  the  nucleus;  the  structure  of  the  cytoplasm  is  regarded  as 
real,  but  as  secondary  and  dependent  upon  the  primary 
structure  of  the  nuclear  elements.  These  nuclear  organs,  the 
chromosomes,  are  thought  to  maintain  a  specific  physiological 
continuity  from  one  generation  of  ova  to  the  next  and  thus 
to  constitute  a  real  physical  basis  of  heredity. 

These  two  general  hypotheses  have  in  common  the  idea  of  a 
fixed  promorphological  structure  within  the  germ  which  becomes 
expressed  epigenetically.  They  differ  as  regards  the  particular 
part  of  the  germ  whose  promorphology  is  to  be  regarded  as 
primary,  and  yet  it  is  quite  possible  that  both  hypotheses 
contain  elements  of  truth.  It  remains  now  for  us  to  review 
some  of  the  more  significant  facts  of  development  bearing 
directly  upon  these  ideas  in  order  to  determine,  perhaps  which, 
perhaps  how  much  of  each,  is  justified.  In  doing  this  we  shall 


266  GENERAL  EMBRYOLOGY 

not  attempt  to  relate  the  evidence  directly  to  either  hypothesis, 
leaving  that  for  the  reader;  and  in  conclusion  we  shall  attempt  a 
summary  which  may  serve  to  bring  the  two  hypotheses 
together. 

We  may  first  describe  certain  facts  associated  chiefly  with  the 
hypothesis  of  germinal  cytoplasmic  localization. 

In  the  first  place  it  is  perfectly  clear  that  the  ovum  does 
possess  a  marked  structure  and  organization,  indicated  in 
several  ways.  Occasionally  the  ovum  may  show  external 
differentiations  of  form;  such  an  egg  as  that  of  the  squid 
(Loligo,  Fig.  113)  or  the  fly  (Musca,  Fig.  47),  is  obviously  not 
only  bilaterally  symmetrical  but  it  exhibits  definite  antero- 
posterior  and  dorso- ventral  differentiation.  In  a  few  instances 
the  eggs  of  a  species  are  dimorphic,  and  while  apparently  the 
nuclei  of  both  kinds  are  identical  in  structure,  the  total 
volume  of  one  form  may  be  three  times  that  of  the  other.  One 
of  the  very  important  and  highly  significant  factors  in  the 
organization  of  the  ovum  is  that  of  polarity,  already  described 
(e.g.,  Figs.  42,  93).  In  many  cases  the  polarity  of  the  ovum 
can  be  traced  back  into  oogonial  stages,  where  it  is  seen  to 
correspond  with  the  polarity  of  the  cells  of  the  germinal  epithe- 
lium (Mark).  Polarity  pervades  the  whole  structure  of  the 
mature  ovum  and  is  expressed  in  a  variety  of  ways,  by  the 
eccentric  position  of  the  nucleus,  by  the  point  at  which  the 
polar  bodies  are  formed,  by  the  disposition  of  the  deutoplasm, 
and  by  the  arrangement  and  distribution  of  a  variety  of  formed 
substances,  such  as  pigments,  granules,  and  vacuoles  of  many 
kinds. 

Living  protoplasm,  as  we  have  seen,  consists  of  a  fundamental 
matrix  or  ground  substance  of  rather  uncertain  form  and  com- 
position, and  suspended  within  this,  particles  and  granules 
of  many  sizes,  forms,  and  materials,  the  nature  of  which  gives 
character  to  a  particular  region  of  protoplasm.  These  different 
kinds  of  substance  are  not  distributed  at  random  through  the 
ovum,  but  they  are  localized  in  certain  regions,  as  zones  or 
layers,  either  horizontal  or  concentric. 

There  are  at  present  two  views  as  to  the  relation  of  these 


DIFFERENTIATION,  HEREDITY,  SEX  267 

substances  to  the  fundamental  polarity  and  general  organization 
of  the  egg.  In  the  opinion  of  some  these  substances  are  truly 
to  be  regarded  as  the  initial  developmental  differentiations  of 
the  ovum.  Each  of  these  substances  has  a  specific  function 
in  development  and  leads  to  the  formation  of  a  certain  tissue  or 
organ  alone.  Consequently  these  materials  are  known  as 
"  organ-forming  substances."  Before  cleavage  they  usually 
assume  a  very  definite  symmetry  of  distribution,  closely 
related  to  that  of  the  later  developing  organism,  and  during 
cleavage  they  become  distributed  among  certain  specific  groups 
of  cells  whose  lineage  can  be  traced  directly  to  the  rudiments 
of  certain  organs. 

Thus  in  the  sgg  of  the  Ascidian,  Cynthia  (Styela),  fully 
described  by  Conklin  and  one  of  the  best  examples  of  this  type 
of  structure,  at  the  close  of  the  first  cleavage  there  are  five 
regions  of  protoplasm,  present  in  amounts  roughly  proportional 
to  the  size  of  the  parts  to  which  they  later  give  rise,  and 
distinguishable  by  the  character  of  their  granular  contents 
(Fig.  92).  At  the  animal  pole  is  a  superficial  region  of  com- 
paratively clear  protoplasm,  the  ectoplasm,  from  which  the 
ectoderm  develops;  in  the  vegetal  pole  there  is  a  dark  gray 
region,  the  endoplasm,  rich  in  yolk  and  later  forming  the 
endoderm;  the  mesoderm  is  formed  from  a  crescentic  region, 
the  mesoplasm,  located  just  below  the  equator,  on  the  posterior 
side;  this  is  characterized  by  its  content  of  yellow  pigment,  and 
is  divided  into  lighter  and  darker  areas,  forming  respectively 
the  mesenchyme  and  the  tail  muscles  (myoplasm);  a  light 
gray  crescent  around  the  anterior  border  forms  later  the  neural 
plate  and  notochord  (neuroplasm,  chordaplasm). 

These  substances  are  arranged  symmetrically  with  reference 
to  the  first  cleavage  plane  and  this  corresponds  also  to  the 
median  plane  of  the  larva  and  adult,  and  thus  from  the  first 
separates  right  and  left  sides  of  the  body.  The  second  cleavage 
plane,  at  right  angles  to  the  first,  separates  the  yellow  meso- 
plasm from  the  light  gray  neuroplasm  and  chordaplasm,  and 
the  third  cleavage,  horizontal,  separates  the  clear  ectoplasm 
from  the  other  substances.  The  later  cell  lineage  of  this  form 


268 


GENERAL  EMBRYOLOGY 


D 


FIG.  123. — Changes  in  the  structure  of  the  egg  of  the  Annulate,  Choetopterus, 
during  and  after  fertilization.  From  Lillie.  A.  Axial  section  through  fully 
grown  oocyte,  still  within  ovarian  epithelium.  Ectoplasm  in  upper  two-thirds  of 
egg  only.  B.  Axial  section  through  primary  oocyte,  ten  minutes  after  extrusion, 
three  minutes  after  fertilization.  Ectoplasm  has  already  flowed  to  the  vegetal 
pole,  leaving  an  exposed  area  of  endoplasm  at  the  animal  pole.  Part  of  the  a 


DIFFERENTIATION,  HEREDITY,  SEX  269 

is  fully  described  by  the  same  author  and  it  is  clear  that  each 
of  these  substances  becomes  contained  within  the  cells  forming 
the  rudiment  of  the  particular  organ  or  tissue  mentioned. 

There  are  few  other  instances  known  where  it  is  quite  so 
easily  possible  to  distinguish  the  specific  formed  substances  in 
the  egg.  But  in  many  ova,  before  cleavage  it  is  possible  to 
distinguish  several  kinds  of  substance;  thus  there  are  known 
in  the  eggs  of  certain  Echinoderms  four  differentiated  materials 
(Lyon),  three  in  Hydatina  (Whitney),  Dentalium  (Wilson), 
Physa,  Lymncea  (Conklin),  four  in  Cumingia  (Morgan),  three 
in  the  frog  (McClendon),  etc.  (Figs.  42,  45,  86,  91,  123,  126, 
129). 

The  term  "  organ  forming"  as  applied  to  these  materials  does 
not  rest  alone  upon  the  observation  of  normal  development, 
but  upon  experimental  grounds  as  well.  For  an  example  of  this 
kind  of  evidence  we  may  return  to  the  work  of  Conklin  on  the 
egg  of  Cynthia.  If  one,  say  the  right,  of  the  blastomeres  of 
the  two-cell  stage  is  injured  so  as  to  prevent  its  further  develop- 
ment, the  remaining  blastomere  develops  as  it  would  normally, 
i.e.,  into  the  left  half  of  an  embryo  and  larva,  containing  approxi- 
mately one-half  the  number  of  cells  found  in  the  normal 
organism  at  the  corresponding  stage  (Fig.  124).  Embryos 
derived '-from  ti^two  anterior  cells  of  the  four-cell  stage  never 
possess  a  tail  or  any  muscle  .cells,  while  chorda  cells  and 
neural  plate  cells  differentiate  normally,  and  the  ef^oderm  and 
^covering  ectoderm  ar$  afteajt^pically  formed  (Fig>  125).  Corre- 
spondingly, embryos  delved  from  fh£  two. posterior  cells  have 
no  chorda,  nerve,  or  sensory  cells,  or  gastral  endodernvbut 

endoplasm  has  also  passed  to  the  vegetal  pole.  The  germinal  vesicle  has  broken 
down,  and  the  maturation  spindle  is  in  the  process  of  formation,  between  the 
two  asters.  The  residual  substance  of  the  germinal  vesicle  is  clearly  seen. 

C.  Axial  section  through  secondary  oocyte,  thirty-two  minutes  after  fertilization. 

D.  Longitudinal  section,  first  cleavage;  late  anaphase.     Posterior  end  toward  the 
left,  anterior  toward  right.     The  ectoplasm  of  the  polar  lobe  has  been  separated 
from  the  remainder.     E.  Sagittal  section  through  stage  of  about  sixty-four  cells. 
The  small  upper  cells  are  the  apical  cells.     The  ectoplasmic  defect  will  be  noted 
in  the  posterior  apical  cell  to  the  observer's  right.     A,  animal  pole;  c,  chromatin; 
c.v.,  chromosomal  vesicles  of  daughter  nuclei;  E,  ectoplasm;  e.a.,  e.b.,  e.c.,  endo- 
plasm a,  6,  and  c.     E.d.,  ectoplasmic  defect;  En,  endoderm  cells;  m,  mesoblast 
cell;  n,  nucleolus;  p.L,  polar  lobe;  r.s.,  residual  substance  of  germinal  vesicle; 
s.a.,  sperm  aster;  s.n.,  sperm  nucleus;  V,  vegetal  pole;  X,  derivatives  of  first 
somatoblast. 


270 


GENERAL  EMBRYOLOGY 


FIG.  124. — The  development  of  one  of  the  blastomeres  of  the  two-cell  stage  of 
the  Tunicate,  Cynthia.  From  Conklin.  The  yellow  material  (see  Figs.  91,  92) 
is  stippled;  the  boundary  between  clear  protoplasm  and  yolk  is  indicated  by  a 
crenated  line. 

A.  Right  half  of  eight-cell  stage;  posterior  view.  B.  Right  half  of  thirty-cell 
stage;  dorsal  view.  C.  Right  half  of  forty-eight-cell  stage;  dorsal  view.  D. 
Right  half  of  sixty-four-  to  seventy-six-cell  stage;  dorsal  view.  E.  Right  half 
of  gastrula  of  about  220  cells  derived  from  four-cell  stage.  The  neural  plate, 
chorda,  and  mesoderm  cells,  are  present  only  on  the  right  side,  and  in  their 
normal  positions  and  numbers.  F.  Right  half  of  young  tadpole;  dorsal  view. 
Derived  from  four-cell  stage.  The  notochord  consists  of  a  small  number  of  cells 
which  are  interdigitating;  muscle-cells  and  mesenchyme  lie  on  the  right  side  of. 
the  chorda,  but  not  on  the  left  side,  though  the  muscle  cells  have  begun  to  grow 
around  to  the  left  side.  The  neural  plate  is  normal  in  position  but  not  in  form. 
m'ch.,  mesenchyme;  ms,  muscle  cells. 

The  cell  nomenclature  in  this  and  the  following  figure,  differs  from  that  de- 
scribed in  Chapter  VI.  The  right  and  left  halves  of  the  embryo  are  designated 


DIFFERENTIATION,  HEREDITY,  SEX  271 

consist  of  a  mass  of  muscle  and  mesenchyme  cells  with  a 
double  row  of  caudal  endoderm  cells,  as  in  the  corresponding 
region  of  a  normal  larva.  Equivalent  results  may  be  obtained 
by  injuring  one  or  three  cells  of  the  four-cell  stage  (Fig.  125). 

The  work  of  Roux,  Fischel,  Wilson,  and  many  others  has 
demonstrated  similar  localizations  in  the  eggs  of  many  forms — • 
the  frog,  other  Ascidians,  several  Molluscs,  Annulates,  and  the 
Ctenophores,  but  we  must  limit  ourselves  to  the  mention  of 
only  a  few  interesting  details  of  the  experiments  on  these 
forms. 

The  Mollusca  afford  several  very  striking  illustrations  of 
the  effects  of  the  removal  of  parts  of  the  egg  or  of  blastomeres. 
The  egg  of  Dentalium,  as  described  by  Wilson,  has  an  upper 
clear  area  which  normally  forms  the  ectoderm,  a  middle  reddish 
or  brownish  pigmented  zone  forming  endoderm,  and  a  lower 
clear  area  which  during  cleavage  forms  a  peculiar  "yolk  lobe" 
or  "polar  lobe"  (Fig.  126).  When  this  yolk  lobe  is  entirely 
removed  from  the  segmenting  egg  the  development  of  the 
remainder  proceeds  as  if  it  were  present,  and  a  larva  is  formed 
which  lacks  the  apical  organ  and  the  entire  post-trochal  region 
(for  explanation  of  terms  see  Fig.  126),  and  which  develops 
later  into  an  organism  lacking  those  structures  which  would 
normally  have  been  formed  from  this  part  of  the  egg  and  larva, 
namely,  the  foot,  mantle,  shell  glands  and  shell,  pedal  ganglion, 
and  apparently  also  coelomic  mesoblast.  Other  Mollusca  give 
essentially  similar  results  although  of  course  not  all  possess  a 
yolk  lobe;  but  removal  of  blastomeres  is  always  followed  by 
absence  of  specific  parts  in  later  development  (e.g.,  Wilson, 
Crampton,  Conklin)  (Fig.  126). 

The  blastomeres  of  several  species  of  animals  fall  apart,  or 
may  be  shaken  apart  easily,  after  a  brief  treatment  with  calcium- 
free  sea  water,  a  fact  discovered  by  Herbst  and  applied  by  him 

by  the  same  letters,  those  referring  to  the  right  side  being  underscored.  A  and  B 
refer,  respectively,  to  the  anterior  and  posterior  hemispheres.  After  the  third 
cleavage,  all  cells  lying  on  the  polar  body  side  of  that  cleavage  plane  are  desig- 
nated by  lower  case  letters,  while  those  on  the  opposite  side  of  that  plane  continue 
to  be  designated  by  capitals.  The  first  exponent  following  a  letter  indicates  the 
generation  to  which  the  cell  belongs.  The  second  exponent  refers  to  the  position 
of  the  cell  relative  to  the  vegetal  pole. 


272 


GENERAL  EMBRYOLOGY 


and  many  others  to  an  analysis  of  this  problem  of  localization. 
The  blastomeres  of  many  Echinoderms,  Molluscs,  etc.,  can  be 
thus  separated,  and  it  is  a  remarkable  fact  that  one  of  two,  four, 


i§ch. 


v.end/ 


FIG.  125. — The  development  of  blastomeres  of  the  four-cell  stage  of  Cynthia- 
From  Conklin.  A.  Anterior  half-embryo  derived  from  two  anterior  blastomeres. 
The  yellow  crescent  remains  visible  in  the  posterior,  uninjured  cells  (B3).  Sense 
spots  are  present  but  the  neural  plate  never  forms  a  tube.  The  chorda  cells  lie 
in  a  heap  at  the  left  side.  There  is  no  trace  of  muscle  substance  or  of  a  tail. 
B.  Posterior  half-embryo  from  the  two  posterior  blastomeres.  Dorsal  view, 
focussed  deeply  upon  the  double  row  of  ventral  endoderm  cells  in  the  mid-line, 
a  mass  of  mesenchyme  cells  on  each  side.  No  neural  or  chorda  cells.  C.  Left 
anterior  quarter  embryo  from  cell  A ;  dorsal  view.  An  invagination  of  the 
ectoderm  cells  has  the  appearance  of  a  gastrula,  but  is  probably  the  invagination 
of  the  neural  plate.  D.  Left  anterior,  and  right  posterior  quarter-embryos, 
from  cells  A  and  B;  dorsal  view.  The  former  shows  thickened  ectoderm  cells, 
probably  neural  plate,  around  the  endoderm  cells;  in  the  latter  are  eight  muscle 
cells  and  three  caudal  endoderm  cells,  irich,  mesenchyme;  ms,  muscle  cells; 
n.p.,  neural  plate;  v.end.,  ventral  endoderm. 


eight,  or  even  one  of  sixteen  cells,  continues  to  develop  for 
some  time  and  forms  those  parts,  and  only  those,  which  it 
would  have  formed,  had  development  of  the  entire  cell  group 


DIFFERENTIATION,  HEREDITY,  SEX 


273 


Gr 


FIG.  126.  —  Development  of  the  Mollusc,  Dentalium,  after  removal  of  the 
"polar  lobe."  From  Wilson.  A.  Egg  twenty  minutes  after  extrusion,  and 
before  maturation  is  completed,  showing  regional  differentiation.  B.  Section 
through  egg  one  hour  after  fertilization,  showing  the  beginning  of  the  formation 
of  the  polar  lobe.  C.  Normal  eight-cell  stage,  viewed  from  lower  pole.  The 
polar  lobe  is  the  light  part  of  cell  D.  D.  Normal  sixteen-cell  stage  viewed  from 
lower  pole.  The  materials  of  the  polar  lobe  are  now  contained  in  the  cell  marked 
X.  E.  Sixteen-cell  stage  of  egg  from  which  the  polar  lobe  was  removed  during 
the  first  cleavage  period.  F.  Normal  trochophore  of  twenty-four  hours.  G. 
Trochophore  of  twenty-four  hours,  developed  from  "lobeless"  egg.  H.  Normal 
larva  of  seventy-two  hours,  showing  foot  and  shell.  /.  Seventy-two-hour  larva 
from  "lobeless"  egg.  p,  polar  lobe. 


274 


GENERAL  EMBRYOLOGY 


FIG.  127. — Cleavage  of  isolated  blastomeres  in  the  egg  of  the  Mollusc,  Patella. 
From  Wilson.  A-D,  X  167;  G,  H,  X  242;  others  X  208.  A.  Normal  eight-cell 
stage,  viewed  from  upper  pole.  Fourth  cleavage  in  progress.  B.  Normal  thirty- 
two  cell  stage,  from  side.  C.  The  so-called  "ctenophore  stage"  (normal)  viewed 
from  upper  pole.  The  primary  trochoblasts  are  ciliated.  D.  Normal  trocho- 
phore  of  thirty  hours,  from  left  side.  Body  wall  in  section,  prototrochal  cells  in 
surface  view.  E.  Second  cleavage  of  an  isolated  micromere  of  the  first  quartet 
(one  of  eight  cells).  F.  Entire  quadrant — products  of  first  and  second  quartet 
cells,  being  formed  much  as  in  the  normal  egg.  G.  Larva  of  twenty-four  hours 
from  one  of  eight  cells  (micromere).  From  side,  showing  trochoblasts  below, 
apical  cells  above.  H.  Product  of  primary  trochoblast  isolated  from  sixteen- 
cell  stage.  /.  First  division  of  isolated  first  quartet.  J.  Division  of  isolated 
basal  cell  of  eight-cell  stage,  showing  typical  arrangement  of  these  four  cells  as 
in-  the  normal  group  of  thirty-two.  K.  Larva  of  twenty-four  hours,  developed 
from  group  like  /,  showing  two  secondary  trochoblasts  and  two  feebly  ciliated 
cells  (?  pre-anal  cells),  m,  primary  mesoblast  cell;  s.g,  shell  gland. 


DIFFERENTIATION,  HEREDITY,  SEX  275 

been  occurring  normally,  although  ultimately  a  normal  larva 
may  be  formed  (Fig.  127). 

The  idea  that  these  differentiated  materials  of  the  cytoplasm 
really  play  the  role  of  organ-forming  substances  in  develop- 
ment, is  opposed  by  some  (Lillie,  Morgan,  and  others).  The 
opposed  idea  rests  upon  the  experimental  evidence  that, 
briefly  stated,  the  really  primary  and  fundamental  organization 
of  the  egg  cytoplasm  concerns  the  ground  substance  of  the 
protoplasm;  the  arrangement  of  the  various  formed  stuffs 
coincides  with  a  similar  and  primary  polarity  and  organization 
of  this  fundamental  protoplasmic  matrix.  This  structure  is 
less  manifest,  but  is  really  the  factor  which  determines  the 
arrangement  of  the  suspended  cytoplasmic  and  deutoplasmic 
granules  and  vacuoles.  The  correspondence  between  the 
arrangement  of  these  stuffs  and  the  organ-forming  substances 
proper,  is  thus  unessential,  for  the  localization  of  the  germ  is 
primarily  a  localization  of  the  ground  substance.  In  other 
words,  the  varieties  of  material  described  by  Conklin  in  Cynthia, 
for  example,  are  only  secondarily  related  to  the  later  differen- 
tiation of  particular  organs  or  tissues,  and  their  arrangement 
is  dependent  upon  the  same  primary  factor  that  determines  the 
arrangement  of  the  organs  and  tissues. 

The  evidence  for  this  view  is  found  chiefly  in  the  results  of 
certain  experiments  upon  the  eggs  of  Chcetopterus  (Lillie)  and 
Arbacia  (Morgan  and  Spooner).  The  granules  which  give 
character  to  the  various  regions  of  the  cytoplasm  differ  in 
specific  density,  and  consequently  can  be  thrown,  by  centrifu- 
gal force,  into  abnormal  regions  of  the  ovum.  When  this 
is  done  normal  cleavage  and  development  may  proceed,  normal 
with  respect  to  the  original  polarity  of  the  ovum  and  not  with 
respect  to  the  new  polarity  as  indicated  by  the  altered  arrange- 
ment of  the  plasmas. 

To  illustrate,  the  egg  of  the  sea-urchin  Arbacia,  contains  four 
different  kinds  of  substance;  one  of  these  is  distinguished  by 
the  presence  of  bright  orange  or  reddish  pigment.  In  normal 
development  this  substance  lies  toward  the  lower  pole  and 
becomes  localized  in  the  lower  quartet,  so  that  when  the  micro- 


276 


GENERAL  EMBRYOLOGY 


meres  form  here,  they  are  composed  of  this  material.  The 
micromeres,  which  later  form  the  mesenchyme,  always  appear 
at  the  pole  opposite  the  micropyle  (Fig.  109),  which  marks  the 
point  of  attachment  of  the  ovum  in  the  ovarian  germinal 
epithelium;  this  is  also  the  point  at  which  gastrulation  com- 
mences. The  centrifuge  brings  about  a  stratification  of  these 
substances  which  is  independent  of  the  polarity  of  the  ovum, 
since  the  ovum  may  assume  any  position  with  reference  to  the 


FIG.  128. — Normal  cleavage  in  the  sea-urchin,  Arbacia,  following  abnormal 
distribution  of  egg  substances  by  centrifuging.  From  Morgan  and  Spooner. 
The  figures  are  turned  so  that  the  pigment  (dotted  area)  is  downward.  The 
location  of  the  cleavage  planes,  and  the  position  of  the  micromeres,  which  always 
mark  the  invaginating  pole  also,  are  independent  of  the  induced  stratification  of 
the  egg  substances. 

axis  of  rotation  of  the  machine.  The  pigmented  protoplasm 
may  be  thrown  to  any  part  of  the  cell.  But  Morgan  has  found 
that  the  cleavage  of  eggs  with  abnormally  distributed  sub- 
stances proceeds  normally  with  reference  to  the  original 
polarity  of  the  ovum  and  not  according  to  the  induced  arrange- 
ment. The  micromeres,  for  example,  continue  to  form 
opposite  the  micropyle,  and  gastrulation  occurs  here,  as  usual, 
although  the  pigmented  protoplasm  may  occupy  some  remote 
and  unusual  position  in  the  cell  (Fig.  128).  Perfectly  typical 
larvae  develop  from  such  eggs,  normal  save  in  the  distribution 
of  pigment.  Development  and  differentiation  thus  seem  to  be 
quite  independent  of  the  so-called  "  formative  stuffs,"  which 
are,  in  such  instances,  evidently  not  "  organ  forming." 


DIFFERENTIATION,  HEREDITY,  SEX 


277 


Several  other  forms  are  known  to  give  similar  results.  One 
of  the  clearest  instances  is  to  be  seen  in  the  Lamellibranch, 
Cumingia,  also  described  by  Morgan.  The  egg  of  Cumingia 
contains,  besides  the  clear  protoplasm,  three  kinds  of  formed 
substance,  yolk,  pigment,  and  oil.  With  the  centrifuge  these 
can  be  thrown  to  any  part  of  the  cell  whatever,  and  yet  cleavage 
and  development  proceed  normally  with  reference  to  the 


FIG.  129. — Normal  development  of  the  Pelecypod,  Cumingia,  following  ab- 
normal arrangement  of  the  egg  substances  by  centrifuging.  After  Morgan.  The 
pigment  i§  indicated  by  stipples,  the  oil  by  small  circles.  A.  Two-cell  stage  with 
oil  in  small  cell.  B.  Same  with  oil  in  large  cell.  C.  Same  with  oil  in  both  cells. 
D.  Normal  trochophore,  showing  usual  distribution  of  pigment  and  oil.  E. 
Trochophore  with  oil  on  oral  side,  and  yet  normal.  F.  Normal  trochophore  with 
oil  aboral  and  interior. 

original'- polarity  and  not  at  all  to  the  actual  distribution  of 
these  substances  (Fig.  129).  It  should  b^noteS  tlmt,  Although 
this  has  not  been  definitely  ^determined  fo^  Cumingia,  the 
Mollusca  in  general, are  excellent  examj^esf  of  determinately 
cleaving  eggs  and  the  removal  rpf  parts  of  ova  is  followed  by 
definitely  corresponding  defects  in  embryo  amUferva.  '  *••  • 
Such  experiments  as  these  seem  to  indicate  clearly  that  the 


278  GENERAL  EMBRYOLOGY 

determining  structure  of  the  ovum  is  really  that  of  the  under- 
lying protoplasmic  ground  substance,  and  that  the  arrange- 
ment of  the  various  formed  substances  coincides  with  this,  is 
determined  by  it  in  the  first  place,  but  is  not  always  or  neces- 
sarily concerned  directly  in  the  later  differentiations  of  the 
ovum.  The  defects  following  removal  of  parts  of  the  unseg- 
mented  ovum,  or  of  blastomeres  result  therefore  from  the  loss 
of  parts  of  this  underlying  structure,  and  not  from  the  loss  of 
the  formed  materials  or  "  formative  stuffs,"  which  in  such 
cases  at  least  turn  out  not  to  be  "formative." 

Lillie  has  shown  that  carefully  graduated  centrifuging 
reveals  the  existence  in  the  egg  of  Chcetopterus,  of  certain 
regional  differentiations  of  the  ground  substance,  indicated  by 
the  differences  in  the  ease  with  which  the  granules  of  various 
sizes,  and  other  structures,  such  as  parts  of  the  mitotic  figure, 
pass  through  it.  These  regions  are  not  otherwise  visible  but 
Lillie  suggests  that  since  they  are  undoubtedly  real  they  may 
represent  or  mark  in  some  way  the  primary  organization  of  the 
cytoplasm. 

According  to  this  view  of  organization  the  term  "  organ- 
forming  substances"  for  the  visibly  differentiated  substances 
is  a  misnomer.  For  if  these  are  removed  to  abnormal  positions 
within  the  cell  they  are  then  not  related  to  the  formation  of  the 
same  structures  that  they  are  in  normal  development. 

At  present  it  seems  difficult,  though  not  impossible  as  we 
shall  see  later,  to  reconcile  these  two  views  as  to  the  real  seat 
of  the  primary  organization  of  the  cytoplasm  of  the  ovum;  the 
balance  of  evidence  appears  to  favor  the  conception  of  organi- 
zation as  a  condition  of  the  fundamental  ground  substance  of 
protoplasm.  But  in  any  event  it  is  perfectly  clear  that  the 
cytoplasm  is  organized  definitely. 

We  should  call  attention  in  passing  to  the  fact  that  many  of 
the  results  described  above  indicate  that  cleavage  is  not  to  be 
regarded  always  as  a  developmental  process  of  primary  impor- 
tance. Conklin  has  called  attention  to  the  fact  that  in  Cynthia 
the  early  cell  boundaries  do  not  always  coincide  with  the  limits 
of  the  various  kinds  of  cytoplasm.  The  determination  of  the 


DIFFERENTIATION,  HEREDITY,  SEX 


279 


structure  of  the  egg  and  the  localization  of  these  materials, 
precede  cleavage  and  are  independent  of  it.  Certain  pressure 
experiments  to  be  mentioned  shortly,  illustrate  the  independ- 
ence of  cleavage  and  determination,  and  the  centrifuging 
experiments  similarly  demonstrate  the  independence  of 
cleavage  and  the  distribution  of  the  cytoplasmic  stuffs.  Indeed 
Lillie  describes  the  formation  of  a  trochophore-like  larva  from 
the  egg  of  Chcetopterus  in  which  cleavage  had  been  artificially 
prevented;  this  embryo  formed  external  cilia  and  certain  other 


FIG.  130. — Development  and  differentiation  in  the  absence  of  cell  division,  in 
Chcetopterus.  From  Lillie.  A,  B,  C.  Ciliated,  uninucleated  unsegmented  eggs, 
about  twenty-three  hours  old.  The  vacuoles  are  about  in  the  position  of  the 
prototroch  of  the  larva.  D.  Ciliated  unsegmented  egg  about  twenty-eight  hours 
old;  most  of  the  endoplasm  has  been  consumed,  e,  endoplasm. 

differentiated  structures  in  the  complete  absence  of  cell  divisions 
(Fig.  130).  The  normal  processes  of  development  are  varied 
and  more  or  less  independent  of  each  other,  while  having 
common  reference  to  some  general  underlying  condition. 

We  must  now  consider  the  facts  of  development  in  a  consider- 
able group  of  eggs  which  do  not  show  any  such  results  as  those 
described  in  the  foregoing  pages.  Although  these  eggs  possess 
more  or  less  differentiated  regions  of  cytoplasm,  yet  removal  of 


280 


GENERAL  EMBRYOLOGY 


parts  or  of  blastomeres  is  not  followed  by  any  structural  defect. 
For  example,  the  blastomeres  of  many  Ccelenterates  (Haeckel, 
Zoja,  Maas,  Wilson)  may  be  separated  when  in  the  two-,  four-, 
eight-,  or  even,  in  some  cases,  in  the  sixteen-cell  stage,  and 
from  such  isolated  blastomeres  typically  formed  embryos  and 
even  free-swimming  larvae  develop,  normal  in  every  respect 
save  that  of  size,  being  respectively  approximately  one-half,  one 


FIG.  131. — Normal  development  of  one  of  the  blastomeres  of  the  two-cell 
stage  of  the  Hydroid,  Clytia  flamdula.  After  Zoja.  A.  Two-cells.  B.  Four- 
cells.  C.  Eight-cells.  D.  Blastula.  E.  Young  polype. 

fourth,  one-eighth,  or  one-sixteenth  the  normal  size  (Fig.  131). 
This  is  true  to  a  certain  extent  also  of  some  of  the  Teleosts  and 
of  Amphibians,  the  Nemerteans,  and  Echinoderms  (Figs.  132, 
133);  in  the  last  named  forms  even  portions  of  the  blastula  or 
gastrula  (Driesch)  may  give  rise  to  normal  but  diminutive 
larvaB  (Fig.  134).  It  seems  very  apparent  that  if  cytoplasmic 
localization  occurs  at  all  in  such  cases,  it  must  be  of  a  very 
different  kind  from  that  described  above. 

This  is  an  appropriate  place  to  mention  certain  experiments 
of  a  different  kind  bearing  upon  this  same  problem.  Eggs 


DIFFERENTIATION,  HEREDITY,  SEX 


281 


may  be  subjected,  during  their  early  cleavages,  to  deforming 
pressure  so  that  the  planes  of  cell  division  appear  in  abnormal 
relations  to  one  another  and  to  the  egg  as  a  whole  (Hertwig, 
Born).  Thus  in  the  sea-urchin  (Driesch)  the  blastomeres  of 
the  eight-cell  stage  instead  of  forming  a  spheroidal  group  may 
be  forced  into  the  form  of  a  flat  plate  (Fig.  135).  When 
released  from  the  pressure  such  eggs  form  perfectly  typical 


FIG.  132. — Gastrulae  and  plutei  from  isolated  blastomeres  of  the  sea-urchins, 
Echinus  (A-D),  and  Sphcerechinits  (E-G).  After  Driesch.  A.  Gastrula  from 
entire  egg.  B.  Gastrula  from  one  blastomere  of  the  two-cell  stage.  C.  Gastrula 
from  one  blastomere  of  the  four-cell  stage.  D.  Gastrula  from  one  blastomere  of 
the  eight-cell  stage.  E.  Normal  pluteus.  F.  Pluteus  from  one  blastomere  of 
the  two-cell  stage.  G.  Pluteus  from  one  blastomere  of  the  four-cell  stage. 

larvae.  And  even  in  a  form  like  the  Annulate,  Nereis,  whose 
cleavage  is  determinate  and  whose  blastomeres  are  highly 
different iated,  Wilson  has  found  that  when  the  egg,  subjected 
to  pressure,  became  divided  by  vertical  planes  into  a  flat  plate 
of  eight  cells,  each  one  contained  substance  normally  found  only 
in  the  macromeres  of  the  lower  pole;  when  released  these 
eight  cells  divided  into  sixteen,  eight  micromeres  and  eight 
macromeres,  instead  of  into  the  normal  twelve  and  four 


282 


GENERAL  EMBRYOLOGY 


respectively.  And  from  these,  normal  larvae  developed;  the 
eight  macromeres  developed  as  the  normal  four  would  have 
done,  although  under  normal  conditions  four  of  the  eight  cells 
and  nuclei  would  have  formed  the  first  quartet,  giving  rise  to 
the  apical  nerve  cells  and  anterior  band  of  ciliated  cells. 

Furthermore,  the  experiment  of  bringing  about  the  coales- 
cence of  parts  of  two  eggs,  or  even  of  two  complete  eggs,  has 


FIG.  133. — Four  normal  but  diminutive  plutei  from  the  isolated  blastomeres 
of  the  four-cell  stage  of  the  sea-urchin,  Strongylocentrotus.  After  Boveri.  A,  B. 
in  oral  view.  C,  D,  lateral  view. 

been  accomplished  with  Ascaris  (Sala,  Zur  Strassen)  and  with 
the  sea-urchin  (Driesch).  The  result  is  again  the  development 
of  a  normal  larva,  of  very  large  size  when  two  entire  eggs  are 
fused  (Fig.  136).  Even  when  two  blastulas  coalesce  the  final 
result  may  be  a  single  larva,  though  with  some  doubling  of 
parts.  One  especially  interesting  point  is  that  normal  develop- 
ment may  result  even  though  the  parts  of  the  two  blastulas 


DIFFERENTIATION,  HEREDITY,  SEX 


283 


may  be  of  different  species,  in  somewhat  different  stages  of 
development,  and  no  micromeres  included  in  the  mass 
(Garbowsky). 


FIG.  134. — Normal  but  diminutive  larvae  of  Echinoderms,  derived  from  por- 
tions of  gastrulae.  From  Jenkinson,  after  Driesch.  a.  Normal  pluteus  of 
Sphcer echinus.  6.  Pluteus  of  same  from  portion  of  gastrula.  c,e.  Normal 
bipennaria  of  Asterias  glacialis.  d,f.  Bipennaria  of  same,  from  vegetative  half 
of  gastrula.  g.  Larva  of  Asterias  with  typical  three-parted  gut,  but  no  coelom, 
from  vegetative  half  of  gastrula,  removed  after  development  of  the  coelomic  sacs. 

While  such  results  as  these  are  at  first  sight  opposed  to  the 
hypothesis  of  germinal  localization,  yet  it  is  quite  possible  to 
reconcile  the  differences  between  such  extreme  forms  as  the 
Echinoderms,  where  one  of  eight  or  sixteen  cells  finally  forms  a 
typical  larva  one-eighth  or  one-sixteenth  normal  size,  and  the 


284 


GENERAL  EMBRYOLOGY 


Ascidian,  where  one  of  four,  eight,  or  sixteen  cells  gives  rise,  not 
to  a  complete  diminutive  larva,  but  to  a  group  of  differentiated 
tissue  cells  of  the  same  kind  that  would  normally  have  been 
formed  from  the  particular  cell,  had  it  remained  in  situ  in  the 
normal  group. 

The  discordance  of  these  results  may  have  one  of  two  mean- 
ings. First,  it  may  mean  that  in  such  eggs  as  those  of  the 
Echinoderms  and  Amphioxus  a  process  of  regeneration  or 


C  D  E 

FIG.  135. — Cleavage  in  the  egg  of  the  sea-urchin,  Echinus  micro-tuberculatus, 
under  pressure.  From  O.  Hertwig,  after  Ziegler.  A,  B.  Eight-  and  sixteen-cell 
stages.  C.  Sixteen-cell  stage  preparing  for  division.  D.  Thirty-two-cell  stage, 
in  the  form  of  a  flat  plate.  E,  Thirty-two-cells  preparing  for  next  division. 
Crosses  mark  cells  in  which  the  spindle  is  vertical  or  oblique,  to  the  plane  of  the 
cell  group. 

regulation  goes  on.  And,  just  as  many  adult  organisms  are 
easily  capable  of  restoring  or  regenerating  lost  parts,  so  the 
embryo  or  even  the  ovum  may  have  the  property  of  reforming 
parts  artificially  removed.  This  process  has  received  the 
special  term  of  post-generation  (Roux).  Such  a  possibility  is 
indicated  by  the  classic  experiment  of  Roux  upon  the  egg  of  the 
frog.  Here,  if  one  of  the  two  blastomeres  is  destroyed  the 


DIFFERENTIATION,  HEREDITY,  SEX 


285 


remaining  one,  if  undisturbed,  develops  into  a  half-embryo; 
but  if  the  egg  is  inverted  after  the  injury  of  one  blastomere, 
then  during  the  consequent  rearrangement  of  the  substances  of 
the  uninjured  cell,  through  the  action  of  gravity,  the  organi- 
zation is  restored  to  the  normal  and  a  small  normal  embryo 
subsequently  develops.  This  shows  that  the  uninjured  half  of 


FIG.  136. — Fusion  of  Echinoderm  larvae.  A,  B.  Sphcerechinus.  After  Driesch. 
C,  D.  Psammechinus  miliaris.  After  Garbowski.  A.  Normal  gastrula.  B. 
Single  gastrula  derived  from  the  fusion  of  two  normal  blastulae,  showing  single, 
large  gut  and  doubled  spicule.  C.  Normal  stage  of  thirty-two-cells.  D.  Organ- 
ism formed  by  the  coalescence  of  parts  of  two  organisms  in  different  stages.  The 
cells  ruled  obliquely  were  part  of  an  eight-cell  stage,  stained  intravitally  with 
neutral  red.  The  remaining  cells  were  part  of  a  normal  thirty-two-cell  stage. 
In  both  C  and  D  the  stippling  marks  the  cells  derived  from  the  vegetative  half 
of  the  egg. 

the  egg  does  possess  the  potentiality  of  developing  as  a  com- 
plete egg. 

Or  second,  the  contrast  between  the  two  extreme  cases 
mentioned  may  mean  that  localization  results  from  a  progres- 
sive process  of  true  development.  Of  course,  in  all  organisms, 
sooner  or  later,  groups  of  cells  become  specifically  differentiated 
as  particular  tissues  and  organs  or  parts  of  organs.  And 
similarly  there  comes  a  tune  in  the  history  of  any  cell  group 


286  GENERAL  EMBRYOLOGY 

when,  once  started  on  its  course  of  differentiation,  return  or 
^differentiation  in  another  direction  is  impossible. 

The  formation  of  localized  germinal  areas  of  cytoplasm  is  to 
be  regarded  as  a  process  of  development,  and  in  the  eggs  of 
different  species  this  process  may  be  carried  forward  at  rela- 
tively different  times  with  respect  to  fertilization,  cleavage,  and 
other  early  developmental  phases.  The  most  important  steps 
in  cytoplasmic  localization  of  the  germ  may  be  completed  while 
maturation  and  fertilization  are  going  on,  prior  to  the  first 
cleavage  (Ascidians);  or  localization  may  be  accomplished 
during  cleavage  (Cerebratulus) ,  or  not  until  the  gastrula  or  post- 
gastrula  stages  (Echinoderms). 

This  idea  is  not  essentially  different  from  that  of  post-genera- 
tion in  certain  respects,  for  regeneration  and  regulation  are 
after  all  essentially  processes  of  development,  deferred  develop- 
ment. The  two  differ  however  in  that,  according  to  the  former 
view  localization  is  really  present  throughout  the  early  stages 
and  disturbances  are  followed  by  an  active  process  of  regula- 
tion; according  to  the  latter,  localization  is  not  determined  dur- 
ing the  earlier  stages  and  when  it  does  appear,  the  parts  of  the 
egg  remaining  after  the  removal  or  injury  of  parts,  behave  as  a 
complete  and  normal  unit,  no  regulation  being  necessary. 

There  is  evidence  for  both  of  these  views,  and  both  may  be 
true  at  the  same  time.  The  second  appears  to  be  the  more 
widely  applicable.  Regulation  seems  more  likely  to  occur 
during  comparatively  late  phases  of  localization.  Evidence  of 
regulation  following  the  separation  of  blastomeres  is  afforded 
by  such  eggs  as  those  of  the  Echinoderms,  where  the  isolated 
blastomere  continues  to  segment  for  a  time  as  if  it  were  part 
of  a  normal  cell  group,  but  gradually  its  cell  products  assume 
the  characters  of  a  typical  whole  group  and  finally  give  rise 
to  a  normal  embryo  and  larva.  In  other  cases  (Amphioxus) 
separated  blastomeres  develop  from  the  beginning'  like  whole 
eggs,  and  no  regulation  is  necessary.  The  results  of  deforma- 
tion by  pressure  also  indicate  that  localization  is  subject  to  a 
regulatory  process  which  may  occur  even  in  a  comparatively 
late  stage  in  cleavage. 


DIFFERENTIATION,  HEREDITY,  SEX  287 

That  the  "  organization "  of  the  cytoplasm  results  from  a 
progressive  developmental  process  is  clearly  evidenced  by  the 
experiments  of  Wilson,  Yatsu,  and  Zeleny  on  the  egg  of 
Cerebratulus  before  cleavage.  If  portions  of  this  egg  are 
removed  before  maturation  has  begun,  while  the  egg  nucleus 
is  still  in  the  form  of  an  intact  germinal  vesicle,  no  defects  are 
seen  in  the  resulting  larva.  Entrance  of  the  'spermatozoon  is 
followed  by  maturation  and  a  general  rearrangement  of  the 
substances  of  the  cytoplasm,  one  result  of  which  is  the  forma- 
tion of  a  cap  of  clear  protoplasm  at  the  animal  pole.  Removal 
of  this  substance  prior  to  or  during  the  first  cleavage,  often  pro- 
duces no  later  abnormality.  The  separated  blastomeres  of  the 
two-cell  stage,  however,  while  for  a  time  cleaving  like  halves, 
soon  assume  the  character  of  wholes.  Those  of  the  four-cell 
stage  continue  longer  to  behave  like  parts,  even  through  the 
blastula  stage,  although  ultimately  they  may  form  typical 
free-swimming  larvaB.  The  degree  of  defect  corresponds  in  a 
general  way  with  the  stage  to  which  cleavage  has  progressed 
at  the  time  of  separation.  Larvae  developed  from  eggs  with- 
out the  upper  quartet,  which  contains  the  clear  protoplasm 
mentioned,  have  typically  formed  enteron,  but  lack  the  apical 
organ.  Larvae  from  this  upper  quartet  have  the  apical  organ 
but  are  without  enteron.  And  the  same  is  true  when,  in  the 
sixteen-cell  stage,  the  upper  and  lower  octets  develop  sep- 
arately. Parts  of  the  blastula  continue  to  develop  for  a  time 
and  form  only  the  restricted  cell  groups  to  which  they  give  rise 
in  normal  development. 

Such  facts  seem  clearly  to  mean  that  cytoplasmic  germinal 
localization  may  be  complete  in  later  stages,  but  incomplete  or 
absent  in  the  earlier,  that  it  is  truly  a  process  or  result  of  devel- 
opment and  not  a  primary  determiner  of  the  course  of  develop- 
ment, not  a  fixed  thing  persisting  from  generation  to  generation, 
which  might  be  regarded  as  the  physical  basis  of  heredity. 

The  conception  of  cytoplasmic  localization  as  a  progressive 
process,  i.e.,  as  one  factor  or  link  in  the  chain  of  developmental 
events,  immediately  raises  the  question  as  to  what  condition 


288  GENERAL  EMBRYOLOGY 

then  lies  back  of  this,  and  determines  the  character  of  the  pro- 
gressive steps  or  reactions.  This  leads  us  directly  to  the  second 
chief  view  as  to  the  fundamental  character  of  the  specific 
organization  of  the  ovum,  that  is,  to  the  hypothesis  of  "nuclear 
analysis"  or  nuclear  determination,  and  to  this  we  may  now 
give  our  attention.  To  state  them  again,  the  essentials  of  this 
hypothesis  are,  that  the  real  germinal  localization  of  the  ovum 
is  to  be  sought  in  the  nucleus,  that  the  organization  of  the  cyto- 
plasm is  preceded  and  its  character  determined  primarily,  by 
the  organization  of  the  nucleus,  that  this  organization  is 
continuous  from  one  generation  to  the  next  and  is  so  to  be 
regarded  as  representing  the  physical  basis  of  heredity.  Polarity 
and  other  cytoplasmic  differentiations,  certainly  exist  in  the 
ovum,  even  before  fertilization  or  cleavage,  but  the  only  struc- 
tural differentiation  of  the  ovum  which  is  invariably  marked 
out  at  all  stages  of  the  organism's  existence,  is  the  differentiation 
between  nucleus  and  cytoplasm.  And  while  not  alone  develop- 
ment, but  all  the  normal  life  processes  of  the  cell  are  the  results 
of  interaction  between  nucleus  and  cytoplasm,  both  being 
essential,  yet  the  action  of  the  nucleus  is  primary  and  seems  to 
determine  the  particularity  of  the  cell  actions. 

This  general  subject  of  nuclear  determination  is  enormously 
complex  and  has  been  the  occasion  for  whole  volumes;  our 
account  of  it  must  perforce  be  brief  and  therefore  more  or  less 
fragmentary  and  dogmatic. 

The  search  for  the  underlying  causes  of  development  is  in 
part  a  search  for  elements  or  conditions  that  are  comparatively 
fixed  and  that  remain  continuous  from  generation  to  generation 
through  the  individual  waves  of  species  life.  Specificity  is 
continuous;  are  there  structural  elements  or  conditions  corre- 
spondingly fixed  and  constant,  not  having  to  develop  anew  in 
each  individual  ontogeny?  Are  there  structures  in  the  germ 
cells  which  determine  the  direction  of  development  and  thus 
represent  (using  this  word  in  a  very  broad  sense)  the  organs  and 
parts  of  the  developing  embryo? 

In  the  endeavor  to  answer  these  questions  the  nuclei  of  the 
germ  cells  at  once  compel  attention  as  containing  organs  whose 


DIFFERENTIATION,  HEREDITY,  SEX  289 

morphology  appears  to  be  constant  and  specific  at  all  stages  of 
the  individual  life  history,  and  through  successive  generations. 
Thus  the  chromosomes  at  once  become  the  foci  of  observation 
and  discussion,  and  the  hypothesis  of  nuclear  determination 
becomes,  to  a  considerable  extent,  the  hypothesis  of  the  specific- 
ity of  the  chromosomes. 

This  conception  has  already  been  outlined  in  Chapter  II;  the 
chromosomes  are  believed  to  be  differentiated  functionally,  in 
a  specific  manner  so  that  each  chromosome  of  the  nucleus 
represents  a  center  of  activity  of  a  particular  character.  That 
is  to  say  each  chromosome,  either  individually  or  as  a  compo- 
nent of  a  unified  group,  determines  a  specific  form  of  reaction 
with  the  cytoplasm,  or  rather  influences  in  a  particular  way 
certain  of  the  reactions  constantly  occurring  between  nucleus 
and  cytoplasm.  And  the  final  result  of  these  reactions  is  the 
production  of  certain  structural  and  physiological  characteris- 
tics of  the  embryo  or  mature  organism.  Thus,  leaving  out  all 
the  intermediate  chain  of  processes  or  reactions,  there  is  an 
actual  correspondence  between  certain  traits  of  the  mature 
organism  and  certain  chromosomal  characters  of  the  gametic 
nuclei.  Of  course  the  chromosomal  characters  determine  only 
the  first  step  in  the  development  of  the  corresponding  trait;  but 
this  in  turn  determines  the  next,  and  so  on.  And  since  the 
quality  of  one  step  or  reaction  in  development  is  determined  by 
the  preceding,  we  are  correct  in  relating  directly  the  character 
of  the  final  steps  in  development  with  the  factor  that  first 
determined  the  trend  of  reaction. 

Emphasis  is  thus  placed  upon  the  physiological  character  of 
the  relation  between  chromosome  and  later  structure,  and  care 
must  be  exercised  constantly,  in  the  discussion  of  this  subject, 
to  guard  against  a  conception  of  this  relation  which  is  too 
strictly,  morphological,  and  which  might  suggest  too  strongly  the 
conception  of  development  as  preformational.  A  wrong  inter- 
pretation of  the  modern  view  of  the  chromosome  relation  leads 
to  a  rather  strict  preformational  view;  but  such  an  idea  does 
not  to-day  represent  the  hypothesis  fairly.  What  is  formed, 
or  preformed,  in  the  germ  is  a  certain  arrangement  or  configura- 


290  GENERAL  EMBRYOLOGY 

tion  of  the  chromatic  substance,  which  in  its  reactions  with  the 
cytoplasm  produces  new  and  specific  conditions,  these  lead  to' 
others,  and  so  on  through  development. 

The  conception  of  the  determinative  character  of  the  chro- 
mosomes must  now  be  modified  to  include  the  idea  that  each 
chromosome  is  not  a  simple  unit,  homogeneous  either  morpho- 
logically or  physiologically.  Each  chromosome  is  to  be  regarded 
as  made  up  of  a  series  or  group  of  elements  which  singly  are 
simple  and  homogeneous,  and  behave  as  physiological  units  or 
"determiners."  These  may  or  may  not  correspond  with  the 
chromioles,  or  granules  of  chromatin,  of  which  the  chromosome 
is  composed;  and  while  it  is  true  that  they  have  never  been 
positively  identified  as  units  or  determiners,  some  such  bodies 
must  be  present  in  the  chromosome  according  to  this  hypothesis. 
Such  determiners,  although  apparently  necessary  hypothetical 
units  cannot  be  described;  they  may  prove  not  to  be  definite 
particles  at  all,  but  rather  dynamic  relations,  or  configurations 
of  substance.  So  for  practical  and  descriptive  purposes  we  are 
nearly  limited  to  the  chromosomes. 

It  is  quite  likely  that  the  chromosomes  may  not  be  the  only 
factors  in  the  determination  of  development,  there  may  be  a 
whole  series  of  factors  back  of  these,  and  we  know  that  a  whole 
series  of  factors  follows  after.  But  if  they  are  proved  to  be 
necessary  links  in  a  chain  of  determining  factors,  then  they  are 
causes  of  differentiation,  and  if  they  are  found  to  be  the  earliest 
visible  differentiations  with  which  later  differentiations  some- 
how correspond,  then  we  may  refer  to  them  as  the  causes  of 
specific  differentiation.  At  some  future  time  it  may  indeed  be 
possible  to  push  the  analysis  of  the  factors  of  differentiation  still 
farther  back;  such  a  possibility  is  in  no  wise  excluded  by  the 
chromosome  hypothesis  as  it  stands  to-day. 

One  of  the  obvious  requirements  of  any  hypothesis  of  differ- 
entiation and  heredity  is  that  it  must  readily  allow  interpreta- 
tion, in  cytological  terms,  of  the  enormously  complex  phe- 
nomena of  alternative  or  Mendelian  heredity.  Most  of  the  traits 
of  an  organism  are  the  property  of  the  species,  common  to  all 
the  individuals  of  a  specific  group.  But  there  are  other  charac- 


DIFFERENTIATION,  HEREDITY,  SEX 


291 


ters  that  are  family  possessions  and  may  or  may  not  be  inherited 
by  individuals.  These  individual  characteristics  are,  in  many 
cases,  comparatively  late  developments.  The  early  characters 
are  those  of  the  larger  group;  those  of  the  species  appear  later, 
and  finally  the  family  and  individual  traits.  The  whole  sub- 
ject of  Mendelism  has  developed  into  an  extremely  complicated 
system,  in  directions  largely  unforeseen.  And  yet  it  is  hardly 
too  much  to  say  that  the  cytology  of  the  germ  cells  and  their 
nuclei  has  on  the  whole  fairly  kept  pace,  and  it  is  in  most  in- 
stances quite  possible  to  parallel  the  facts  of  Mendelism  with  the 
facts  of  chromosome  behavior.  We  shall  return  briefly  to  this 
subject  in  a  more  appropriate  connection. 


« 


FIG.  137. — The  structure  of  chromosomes.  A,  after  K.  C.  Schneider,  others 
after  Bonnevie.  A.  Nucleus  from  epidermis  of  Salamander  larva,  in  telophase. 
B.  Prophase  of  first  cleavage  of  Ascaris  megalocephala  bivalens.  C.  Nucleus 
from  cleavage  stage  of  same.  D.  Inter  kinesis  in  Amphiuma. 

Let  us  now  repeat,  from  this  particular  point  of  view  the 
general  ideas  regarding  the  chromosomes  mentioned  in  Chapter 
II,  emphasizing  certain  topics  and  adding  a  few  details  which 
bear  directly  upon  the  relation  of  the  chromosomes  to  the  proc- 
esses of  development  and  heredity. 

The  chromosomes  are  not  structurally  homogeneous  masses, 
but  are  built  up  of  certain  granules  which  often  have  a  definite 
arrangement  giving  the  chromosome  as  a  whole  a  general 
structure.  This  structure  has  been  variously  described  (Fig. 
137);  in  some  cases  it  seems  to  be  a  cylinder  of  chromatin 


292  GENERAL  EMBRYOLOGY 

granules  with  a  core  of  differentiated  substance,  probably  linin; 
in  other  cases  the  granules  are  arranged  in  a  linear  series  wound 
in  a  definite  spiral;  but  in  still  other  cases  the  granules  seem 
to  have  little  if  any  regular  disposition.  These  granules  are 
very  close  to  the  limit  of  vision,  indeed  often  they  are  invisible, 
and  in  most  cases  it  is  difficult  to  make  confident  assertions 
regarding  their  arrangement.  Of  course  their  invisibility  on 
account  of  minuteness  does  not  prove  that  they  are  not  present. 
Indeed  many,  following  Weismann,  postulate  an  elaborate  series 
of  representative  particles  within  the  nucleus :  the  chromosomes 
or  "idants,"  are  divided  into  "ids"  or  chromatin  granules; 
the  ids  are  then  assumed  to  be  formed  of  groups  of  "determi- 
nants," and  these  in  turn  are  thought  to  be  composed  of  the 
really  elementary,  self-propagating  protoplasmic  units,  the 
"biophores."  Of  thisseries,  the  idants  and  ids  are  visibly  known; 
the  determinants  and  biophores  are  invisible  hypothetical 
bodies,  postulated  to  aid  in  relating  many  of  the  complicated 
facts  of  heredity  to  certain  cytological  facts.  The  assumption 
of  a  final  determining  unit  that  is,  and  seemingly  must  remain, 
invisible  has  proved  fortunate  as  affording  a  convenient  shelter 
against  criticism,  for  such  an  assumption  partly  removes  the 
question  from  scientific  treatment.  We  shall  lose  little  and  gain 
much  by  considering  here  merely  those  elements  of  the  nucleus 
that  can  be  identified  and  whose  behavior  can  be  traced  to 
some  extent,  i.e.,  the  chromosomes  and  chromatin  granules. 

Our  ideas  in  this  field  are,  to  a  remarkable  degree,  the  out- 
growth of  the  pioneer  work  of  Weismann.  Although  based 
upon  the  properties  of  hypothetical  units  whose  behavior  was 
outlined  upon  purely  hypothetical  grounds,  his  conceptions 
of  the  relation  between  chromosome  behavior  and  the  facts  of 
development  and  heredity,  formulated  more  than  thirty  years 
ago,  before  the  science  of  cytology  was  established,  have  a 
distinctly  modern  aspect.  The  remarkable  convergence  of  the 
facts  of  heredity,  of  development,  and  of  cytology,  which  have 
become  known  subsequently  to  the  formulation  of  Weismann's 
hypotheses,  constitutes  splendid  evidence  of  the  keenness  of 
this  great  embryologist. 


DIFFERENTIATION,   HEREDITY,  SEX  293 

We  may  here  suggest  again  the  character  of  the  evidence  for 
regarding  the  chromosomes  of  the  germ  nuclei  of  the  very 
greatest  importance  as  factors  controlling  or  directing  the  proc- 
ess of  development  (i.e.,  the  process  of  heredity),  facts  of  such 
constancy  and  universality  that  they  must  have  some  meaning. 

First  of  all  comes  the  fact  of  the  very  high  degree  of  morpho- 
logical constancy  of  these  organs  throughout  the  tissues  of  the 
species,  not  merely  of  the  individual.  This  constancy,  always 
considering  corresponding  ages  of  course,  concerns  their  number, 
size,  and  form,  and  proves  them  to  be  specific  organs  in  a  real 
sense.  They  are,  with  a  few  easily  explained  exceptions,  pres- 
ent in  pairs  of  similar  elements,  whose  history  can  be  traced 
back  to  their  derivation  hi  groups  of  unpaired  elements  in  the 
male  and  female  gametes.  During  mitosis  the  distribution  of 
the  chromosomes  to  the  daughter  cells  is  never  a  haphazard  proc- 
ess, but  the  whole  process  of  mitosis  appears  to  be  an  adapta- 
tion toward  securing  their  equal  division,  and  the  distribution 
to  the  daughter  nuclei  of  groups  of  similar  morphological 
composition. 

It  is  unnecessary  to  repeat  here  any  of  the  evidence  out- 
lined in  Chapter  II  for  the  idea  of  the  genetic  continuity 
of  the  chromosomes  from  cell  to  cell.  We  saw  there  that  it 
is  the  chromatin  granules  which  may  be  regarded  as  actually 
morphologically  continuous,  and  that  these  may  seem  to 
become  similarly  associated  every  tune  that  the  chromosomes 
visibly  appear.  What  determines  their  constant  association 
in  the  reforming  chromosomes  of  each  cell  generation  is  to  be 
answered  only  hypothetically  if  at  all.  But  in  spite  of  the 
contrary  belief  held  by  some,  the  chromosomes  may  be  regarded 
as  genetically  continuous  individual  elements,  although  the 
details  of  their  composition  may  vary  slightly  from  generation 
to  generation.  And  after  all  it  may  be  that  the  most  important 
continuity  is  that  of  the  chromatin  granules,  supposing  them 
to  be  qualitatively  unlike  and  to  play  the  role  of  specific  de- 
terminers. 

The  validity  of  the  chromosome  hypothesis  has  always  been 
strongly  indicated  by  the  essential  facts  of  syngamy,  namely, 


294  GENERAL  EMBRYOLOGY 

the  formation  of  the  zygotic  nucleus  from  equal  contributions 
of  chromosomes  from  the  male  and  female  parents.  This  is 
the  only  portion  of  the  new  organism  which  is  derived  equally 
from  both  parents,  and  while  it  is  true  that  a  small  amount  of 
cytoplasm  accompanies  the  sperm  nucleus  in  its  entrance  into 
the  ovum,  this  varies  considerably  in  amount  in  different  forms. 
This  latter  fact  together  with  the  general  fact  of  the  primary 
importance  of  the  nucleus  in  all  aspects  of  cell  activity,  com- 
bine to  enhance  the  significance  of  the  equal  derivation  of  the 
chromosomes  (Fig.  106),  especially  in  view  of  the  further  fact 
that  on  the  whole  the  family  and  individual  traits  of  organisms 
are  inherited  with  equal  likelihood  from  either  parent. 

The  behavior  of  the  chromosomes  throughout  the  matura- 
tion process  affords  many  highly  interesting  and  significant 
parallels  between  chromosome  behavior  and  the  facts  of  hered- 
ity. Interest  centers  here  in  the  phenomena  of  synapsis 
and  the  " reducing"  divisions. 

Any  precise  interpretation  of  these  two  phenomena  seems 
impossible  until  more  is  known  with  certainty  regarding  the 
behavior  here  of  the  chromatin  granules,  but  the  phenomena 
themselves  are  readily  interpretable  in  the  light  of  the  facts  of 
alternative,  or  Mendelian  heredity. 

In  synapsis  we  see  the  final  union  of  pairs  of  chromosomes 
introduced  into  a  single  nucleus  at  the  time  of  fertilization,  but 
remaining  distinct  throughout  the  life  of  the  hybrid  generation, 
until  the  time  when  the  hybrid  organism  forms  its  gametes. 
Synapsis  is  not  a  haphazard  junction  of  chromosomes,  but  an 
orderly  union  of  elements  of  paternal  and  maternal  origin, 
similar  in  size,  in  details  of  form,  and  probably  also  in  function. 
The  bivalent  chromosomes  thus  formed  are,  in  consequence  of 
their  derivation  from  two  individuals,  not  quite  homogeneous 
throughout.  Following  synapsis  come  two  divisions  of  each 
chromosome,  and  in  most  organisms  one  of  these  apparently 
divides  the  chromosome  equally,  into  two  similar  parts  (equa- 
tion division),  while  the  other  divides  each  of  the  daughter 
chromosomes  dissimilarly  (reducing  division),  the  dissimilarity 
resulting  from  the  relation  of  the  plane  of  division  to  the  plan 


DIFFERENTIATION,  HEREDITY,  SEX  295 

of  arrangement  of  its  dissimilar  component  granules.  The 
relation  of  the  reducing  division  to  the  chromosome  depends 
upon  the  character  of  the  synapsis,  whether  telosynapsis  or 
parasynapsis,  and  also  upon  the  behavior  of  the  chromatin 
granules  in  all  these  events,  and  it  is  difficult  to  be  certain  of 
this.  It  is  safe  to  say,  however,  that  in  most  cases  each  biva- 
lent chromosome,  composed  in  equal  parts  of  substance  from 
each  parent,  clearly  separates  into  four  elements,  two  having 
one  composition,  two  another.  These  elements  are  then  dis- 
tributed to  separate  gametes,  so  that  with  respect  to  the  com- 
position of  each  separate  chromosome,  the  gametes  produced  by 
an  organism  are  of  two  kinds,  approximately  equal  numerically. 
This  accords  perfectly  with  the  facts  of  Mendelian  heredity, 
upon  the  supposition  that  there  is  a  correspondence  between 
chromatic  elements  and  organismal  traits.  This  may  be  made 
somewhat  clearer  with  the  aid  of  a  diagram:  see  Fig.  80. 

In  the  process  of  maturation,  therefore,  it  is  easily  possible 
to  find  a  mechanism  which  permits  the  segregation  of  charac- 
teristics in  the  germ  cells  and  their  distribution  to  separate 
organisms  in  regular  Mendelian  ratios.  One  important  corre- 
spondence should  not  be  overlooked.  In  Mendelian  heredity 
the  individual  qualities  of  the  parents  may  not  appear  sepa- 
rately until  the  first  generation  after  the  hybrids.  This  is 
possibly  related  to  the  fact  that  the  parental  chromosomes 
undergo  synapsis  and  subsequent  redistribution  first  in  the 
germ  cells  formed  by  the  hybrid,  and  the  segregated  elements 
are,  therefore,  distributed  separately  first  in  the  organisms 
formed  from  these  hybrids,  i.e.,  in  the  Ft  generation. 

The  conclusion  resulting  from  the  study  of  Mendelian  hered- 
ity, that  the  organism  is  a  sum  of  "unit  characters"  which 
in  the  organism  interact  with  one  another,  so  as  to  produce 
a  physiological  whole,  but  which  in  heredity  are  more  or  less 
clearly  separable  units,  affords  strong  evidence  for  the  general 
hypothesis  of  the  representative  particle  composition  of  the 
germ  nuclei.  Chromosomes  might  thus  represent  groups  of 
such  "units"  or  in  occasional  instances  perhaps,  single  units, 
although  this  must  be  the  case  only  rarely,  for  the  total  number 


296  GENERAL  EMBRYOLOGY 

of  unit  characters  is  far  in  excess  of  the  number  of  chromosomes. 

That  the  chromosome  of  the  Metazoan  is  really  made  up  of  a 
group  of  unit  determiners,  is  also  indicated  by  the  behavior  of 
the  Protozoan  nucleus  in  maturation.  In  most  of  the  simpler 
Protozoa  where  the  maturation  phenomena  appear,  there  is  no 
indication  of  definite  elements  like  chromosomes  in  the  nucleus. 
But  in  many  of  the  Ciliates,  in  which  vegetative  chromatin  and 
reproductive  chromatin  become  sharply  separated,  the  latter, 
or  idiochromatin,  is  seen  to  be  formed  into  definite  bodies.  Thus 
in  Paramcecium,  as  observed  by  Calkins  and  Cull  (Fig.  82),  the 
micronucleus  (idiochromatin)  becomes  resolved  into  a  large 
number — more  than  200 — chromatin  granules  (idiochromidia) 
whose  definite  behavior  can  be  traced.  Their  behavior  is  com- 
plex, but  the  result  is  that  each  idiochromidium  is  divided 
longitudinally  and  transversely,  and  the  resulting  daughter- 
bodies  may,  therefore,  be  dissimilar.  After  fertilization  the 
division  of  the  zygotic  nucleus  brings  about  the  division  of  each 
chromidium  and  the  distribution  of  the  halves  to  the  two 
daughter  cells. 

The  very  large  number  of  separate  elements  in  these  "gam- 
etic"  nuclei  may  indicate  that  each  corresponds  with  a  single 
character,  or  with  a  smaller  group  of  characters  than  in  the 
Metazoan,  and  that  therefore  the  chromosome  of  the  Metazoan 
must  be  an  enormously  complex  affair.  All  of  this  lends  weight 
to  the  idea  that  "chromosomes,  the  characteristic  structures  of 
the  nucleus  in  mitosis,  have  had  an  evolution  no  less  surely 
than  has  the  nervous  system,  digestive  system,  or  supporting 
system  of  the  higher  animals,  and  that  the  chromosomes  of  the 
protozoa  have  the  same  relation  to  the  chromosomes  of  the 
metazoa  that  the  organization  of  the  protozoan  body  has  to 
that  of  the  metazoan,  i.e.,  a  unit  structure."  (Calkins,  " Proto- 
zoology," page  171).  Admitting  the  representative  particle 
composition  of  the  chromosomes,  it  must  of  course  follow  that 
their  evolution  in  the  Metazoa,  parallels  the  evolution  of  adult 
form  and  structure. 

If  this  is  true,  then  the  chromosomes  of  the  Metazoan  germ 
cells  must  each  represent  a  congeries  of  determiners,  the  form 


DIFFERENTIATION,  HEREDITY,  SEX  297 

of  association  of  which  might  differ  in  different  species,  as  widely 
as  the  groups  of  characteristics  of  the  adults  differ. 

The  question  as  to  just  how  the  chromatic  determiners  (as- 
suming their  existence)  really  do  affect  the  quality  of  the 
reactions  of  the  developing  organism,  is  still  practically  un- 
touched. To  some  it  seems  necessary  to  postulate  the  asym- 
metrical distribution  of  the  chromatin  granules  through  suc- 
cessive mitoses,  so  that  certain  kinds  of  granules  or  " deter- 
miners" become  distributed  to  certain  cells  and  regions,  directly 
effecting  there  specific  reactions.  No  such  form  of  distribution 
has  been  observed,  though  indeed  it  has  not  been  sought  in  a 
thorough  fashion.  In  tissues  whose  differentiation  is  fairly 
advanced  there  are  certainly  characteristic  and  specific  nuclear 
appearances  which  indicate  that  the  nuclei  as  well  as  the 
cytoplasms  have  undergone  a  real  differentiation,  but  whether 
this  is  related  to  chromosome  or  granule  structure  remains 
undemonstrated. 

If  any  such  sorting  out  of  determiners  occurs  it  must  be  at 
widely  divergent  stages  of  development  hi  the  various  groups, 
on  account  of  the  variety  of  the  results  in  the  way  of  specific 
embryonic  defect  following  the  removal  and  pressure  experi- 
ments described  above.  Indeed  the  results  of  the  pressure 
experiments  referred  to,  become  highly  significant  from  this 
point  of  view,  for  it  will  be  remembered  that  the  presence  of  a 
completely  " foreign"  nucleus  may  in  some  cases  not  influence 
the  particular  form  of  differentiation  of  the  cytoplasm.  To 
say,  in  such  cases  as  these  and  in  the  removal  experiments,  that 
regeneration  may  occur  and  the  proper  determiners  be  reformed, 
does  not  offer  much  that  is  helpful  in  the  way  of  a  solution  of 
this  particular  problem,  for  it  would  necessitate  the  assumption 
of  some  mechanism  back  of  the  " determining"  particles,  by 
which  they  themselves  are  formed  and  determined. 

The  fact  that  parts,  even  small  bits,  of  a  fully  developed  and 
differentiated  organism  may  finally,  through  a  process  of  regu- 
lation, give  rise  to  a  complete  organism  again,  or  that  in  many 
plants,  buds,  bits  of  stem  or  leaf,  may  similarly  give  rise  to  a 
completely  formed  organism  capable  of  developing  typical 


298  GENERAL  EMBRYOLOGY 

germ  cells,  renders  extremely'  unlikely  any  strictly  morpho- 
logical conception  of  the  relation  between  strictly  differentiated 
germinal  determiners  and  the  formation  of  certain  tissues  or 
organs.  The  ideas  cannot  be  overemphasized  or  repeated  too 
often  that,  while  the  thing  or  the  relation  that  we  call  a  deter- 
miner may  sometimes  have  a  morphological  expression  in  the 
germ,  essentially  the  relation  is  physiological — functional — 
probably  chemical  or  energetic  (dynamic),  and  that  the  reac- 
tions or  interactions  of  whole  groups  and  masses  of  particles  or 
systems  are  involved  in  determining  the  intermediate  and  final 
results  of  development. 

One  further  group  of  observations  must  be  considered  in  con- 
nection with  the  possibility  of  the  primary  character  of  the 
nuclear  control  of  development  and  heredity.  During  the 
process  of  development  there  occurs  a  constant  giving  off  of 
substance  from  the  nucleus  to  the  cytoplasm  (Figs.  138,  32). 
At  every  mitosis  only  a  part  of  the  chromatic  substance  is 
formed  into  chromosomes,  while  the  remainder  passes  into  the 
cytoplasm  and  dissolves,  and  of  course  the  whole  fluid  content 
of  the  nucleus,  the  nuclear  sap,  is  discharged  into  the  cytoplasm. 
Herbst  has  emphasized  the  importance  of  the  nuclear  sap  as  an 
important  determining  factor  in  development  and  heredity. 

In  many  instances,  substances  discharged  from  the  nucleus 
into  the  cytoplasm  of  the  oogonia,  especially  during  their 
growth  period,  are  directly  concerned  in  the  formation  of  specific 
materials  and  bodies  of  the  mature  ovum.  And  later  in  develop- 
ment Conklin  has  observed  that  the  cilia  of  the  superficial  cells 
of  Crepidula  develop  only  when  certain  chromatic  granules 
reach  that  region,  a  single  cilium  then  differentiating  opposite 
each  granule.  We  have  already  mentioned  the  fact,  described 
by  Wilson,  that  in  Cerebratulus  the  effects  of  the  removal  of 
parts  of  the  egg  cytoplasm  before  the  germinal  vesicle  has 
broken  down,  are  very  different  from  the  effects  of  the  removal 
of  similar  portions  after  the  contents  of  the  vesicle  have  been 
discharged  into  the  surrounding  cytoplasm  during  the  initial 
stages  of  maturation. 

But  the  evidence  for  the  hypothesis  of  nuclear  determination 


DIFFERENTIATION,  HEREDITY,  SEX  299 


FIG.  138. — A.  Chromatin  extrusion  from  the  nucleus  into  the  cytoplasm  in 
the  oocyte  of  the  Medusa,  Pelagia  noctiluca.  After  Schaxel.  B.  Extrusion  of 
chromatin  into  the  cytoplasm  during  the  maturation  of  the  oocyte  of  Proteus 
anguineus.  After  Jorgensen.  X  1080. 


300 


GENERAL  EMBRYOLOGY 


is  not  altogether  purely  observational;  there  is  some  experi- 
mental evidence  as  well,  although  largely  indirect  and  possibly 
none  is  positively  conclusive. 

For  example,  Boveri  has  described  the  results  of  dispermy  in 
the  sea-urchin  egg.  When  two  spermatozoa  enter  the  ovum  the 
result  is  frequently  the  formation  of  three  or  four  centrosomes 
and  asters  connected  with  one  another  by  spindles,  upon  which 
the  chromosome  groups  are  usually  drawn  in  abnormal  com- 


FIG.  139. — Larva  of  Sphcer echinus,  derived  from  a  dispermic  egg,  showing 
differences  in  nuclear  size,  distribution  of  pigment,  etc.  The  dashed  line  marks 
the  separation  of  the  two  portions  of  the  larva.  After  Boveri  (reconstructed 
from  two  figures),  n,  small  nuclei;  N,  large  nuclei;  p,  pigment. 

binations,  so  that  when  such  an  ovum  cleaves  it  separates 
into  three  or  four  cells  containing  nuclei  whose  composition  is 
therefore  abnormal.  In  many  such  cases  Boveri  finds  that 
each  of  the  three  or  four  cells  forms  a  group  of  cell  descendants 
which  can  be  identified  by  the  presence  or  absence  of  certain 
characters  or  by  unusual  combinations  of  characters  (Fig.  139), 
so  that  the  entire  embryo  may  be  said  to  consist  of  three  or 
four  regions,  each  with  certain  distinctive  characteristics. 
Furthermore,  in  some  instances,  the  cells  of  the  various  fractions 


DIFFERENTIATION,  HEREDITY,  SEX  301 

may  be  characterized  by  unusually  large  or  small  nuclei,  indicat- 
ing the  presence  of  larger  or  smaller  amounts  of  chromatin 
(numbers  of  chromosomes)  than  usual;  the  microscopic  examin- 
ation of  these  multipolar  spindles  shows  that  the  chromosomes 
may  be  distributed  with  great  irregularity  in  the  first  division. 

More  striking  are  the  results  following  the  separation  of  the 
blast omeres  of  such  dispermic  eggs.  The  isolated  cells  of  a 
four-cell  stage  resulting  from  normal  fertilization,  develop 
normally,  producing  four  similar  but  small  normal  larvae 
(Fig.  133).  But  the  isolated  cells  of  one  of  these  three-cell 
stages  develop  dissimilarly,  each  with  certain  defects;  and  just 
as  any  possible  combination  of  chromosomes  may  have  occurred 
in  each  of  the  three  original  cells,  so  all  possible  combinations 
of  characters  are  found  in  the  larvae  developing  from  such  cells 
when  isolated.  Boveri  believes  that  this  warrants  the  conclu- 
sion that,  while  the  presence  or  absence  of  certain  chromosomes 
may  not  result  in  the  presence  or  absence  of  specific  traits,  yet 
a  certain  combination  of  chromosomes  is  essential  for  normal 
development,  a  fact  which  would  mean  only  the  physiological 
specificity  of  the  individual  chromosomes. 

Perhaps  the  most  striking  experimental  results  are  those 
obtained  by  fertilizing  the  eggs  of  one  species,  with  the  sperm 
of  another  species,  genus,  or  even  phylum.  In  the  first  place, 
Boveri  in  1889  reported  that  non-nucleated  egg  fragments  of 
Sphoerechinus  (one  of  the  sea-urchins),  fertilized  with  the  sperm 
of  Echinus,  developed  into  larvae  exhibiting  only  paternal 
characters.  This  appeared  to  afford  strong  evidence  that  the 
characteristics  of  the  nucleus  rather  than  of  the  cytoplasm 
determine  the  course  of  development.  Later  attempts  (See- 
liger,  Morgan,  Boveri)  to  confirm  these  facts  led  to  incon- 
clusive results.  Indeed  exactly  opposed  results  were  obtained 
by  several  investigators  (Driesch,  Loeb,  Godlewski,  Hage- 
doorn).  Eggs  of  one  species  of  Echinoderm  fertilized  with  the 
sperm  of  another  species,  genus  or  class,  of  Echinoderm,  or 
even  with  Molluscan  sperm,  resulted  in  the  development  of 
larvae  possessing  wholly  or  largely  the  maternal  characters. 
These  results  indicated  just  as  strongly  that  the  nuclear  com- 


302 


GENERAL  EMBRYOLOGY 


FIG.  140. — History  of  the  paternal  chromatin  during  the  first  cleavage  in  the 
pseudohybrid  sea-urchin,  Sphcerechinus  9  X  Strongylocentrotus  cT.  After  Herbst. 
A.  First  cleavage  figure.  Sperm  and  egg  pronuclei  associated,  but  not  fused. 
Chromosomes  beginning  to  form  in  the  egg  pronucleus.  B.  Chromosomes  in 
both  pronuclei;  in  separate  groups  with  separate  spindles.  C.  Anaphase  of 
first  cleavage.  Maternal  chromosomes  reaching  the  poles.  Paternal  chromatin 
(chromosomes  no  longer)  forming  an  irregular  mass,  spun  out  on  the  spindle 
between  the  maternal  chromosome  groups.  Z).  Division  completed.  Daughter 
nuclei  reconstructed  and  consisting  entirely  of  maternal  chromatin.  One  of 
the  cells  contains  a  small  vesicle  consisting  of  the  paternal  chromatin,  which  takea 
no  further  share  in  cleavage.  c?f  chromatin  of  the  sperm  pronucleus. 


DIFFERENTIATION,  HEREDITY,  SEX  303 

position  is  of  the  lesser  importance  in  determining  the  devel- 
opment of  specific  traits,  and  of  course  seriously  affected  the 
validity  of  the  hypothesis  of  nuclear  determination. 

These  experiments,  however,  curiously  turned  out  to  afford 
very  strong  evidence  in  support  of  this  hypothesis.  For  other 
workers  (Herbst,  Kupelwieser,  Bataillon)  showed  that  in  many 
of  these,  and  in  other  instances,  the  nucleus  of  the  spermatozoon 
did  not  actually  fuse  with  the  egg  nucleus,  but  remained  either 
partly  or  wholly  inactive,  taking  little  or  no  share  in  the  forma- 
tion of  the  mitotic  figures  of  the  first  and  subsequent  cleavages 
(Fig.  140).  The  resulting  larvaB  therefore  were  not  truly 
hybrids;  the  spermatozoon  had  merely  stimulated  the  egg  to 
develop,  as  hi  artificial  parthenogenesis,  but  itself  took  no  part 
in  the  formation  of  the  nuclear  structures  of  the  larva.  In  the 
absence  of  microscopic  examination  of  the  embryo,  therefore, 
it  is  impossible  to  place  any  emphasis  upon  the  development 
of  purely  maternal  or  paternal  characters  under  such  conditions. 

Fortunately  such  cytological  evidence  is  now  provided 
extensively  through  the  work  "of  Baltzer,  who  has  traced  the 
nuclear  history  of  many  forms  of  Echinoderm  hybrids.  It 
appears  that  part  or  all  of  the  paternal  chromatin,  never  the 
maternal,  may  be  thrown  out  of  the  nuclei  of  such  "hybrids" 
(pseudohybrids).  Such  an  elimination  of  paternal  chromatin 
may  occur  during  the  very  first  cleavage,  or  it  may  be  delayed 
until  the  blastula  or  even  early  gastrula  stage  (Fig.  141).  The 
examination  of  a  long  series  of  hybrids,  showing  all  degrees 
of  purity  of  the  maternal  characters,  leads  Baltzer  to  the  con- 
clusion that  the  degree  to  which  paternal  characters  appear  in 
the  resulting  hybrids,  is  closely  parallel  to  the  relative  amount 
of  paternal  chromatin  which  is  retained  within  the  nuclei  of  the 
organism.  Where  the  fusion  of  the  sperm  and  egg  nuclei 
remains  complete,  the  hybrids  have  intermediate  characters; 
where  little  or  no  chromatin  from  the  spermatozoon  is  retained 
in  the  nuclei,  there  appear,  chiefly  or  alone,  maternal  characters. 
Only  in  the  case  of  the  fertilization  of  the  eggs  of  a  sea-urchin 
(Strongylocentrotus)  with  the  sperm  of  a  Crinoid  (Antedon)  has 
it  been  shown  that  the  fusion  of  the  germ  nuclei  really  occurred 


304 


GENERAL  EMBRYOLOGY 


(Godlewski,  Baltzer)  while  the  larvae  resulting  from  this  cross 
exhibited  certain  characters  which  were  purely  maternal;  but 
this  result  is  wholly  inconclusive  as  evidence  opposing  the  hy- 
pothesis of  nuclear  control. 


FIG.  141. — History  of  the  paternal  chromatin  in  the  pseudohybrids  of  the  sea- 
urchins,  Strongylocentrotus  9  X  Sphcerechinus  6\  After  Baltzer.  A.  Cleavage 
cell  showing  paternal  chromatin  (c?)  outside  the  division  figure.  B.  Early 
blastula.  C.  Late  blastula,  showing  the  elimination  of  the  paternal  chromatin 
in  the  irregular  cells  and  spaces  within  the  blastoccel  (for  normal  blastula  see 
Fig.  109). 

For  it  has  been  found,  in  the  first  place,  that  external  condi- 
tions often  determine  whether  certain  characters  shall  be 
paternal  or  maternal  in  their  qualities  (Vernon,  Tennent). 
Under  certain  conditions  of  temperature,  alkalinity,  etc.,  the 
larva  may  exhibit  paternal  resemblances,  while  under  other 
conditions  maternal  resemblances  may  appear  in  the  same 


DIFFERENTIATION,  HEREDITY,  SEX  305 

cross.  And  in  the  second  place,  the  phenomenon  of  "  domi- 
nance" appears  even  in  these  early  stages  of  development,  and 
a  hybrid  may  show  certain  clearly  maternal  characters  and  yet 
in  other  respects  closely  resemble  the  paternal  type  (Stein- 
bruck,  Driesch,  Boveri,  Loeb  and  Moore).  Great  variability  is 
often  the  rule  and  frequently  it  is  impossible  to  say  whether 
either  parental  trait  really  appears  purely.  It  should  be  pointed 
out,  first,  that  it  frequently  happens  in  Mendelian  inheritance 
that  true  hybrids  are  either  purely  maternal  or  purely  paternal 
with  respect  to  single  traits,  and  second,  that  only  after  synap- 
sis,  which  occurs  in  the  germ  cells  of  the  mature  hybrid  organ- 
ism, are  the  paternal  and  maternal  chromosomes  really  brought 
into  complete  relation. 

On  the  whole,  then,  while  there  are  some  few  results  difficult 
of  favorable  interpretation,  we  may  say  that  the  evidence  from 
hybridization,  though  at  first  distinctly  opposed  to  the  hypothe- 
sis of  nuclear  determination,  at  present  affords  the  strongest 
support  of  this  hypothesis,  and  indicates  that  normally  the 
characters  of  a  hybrid  are  determined  by  both  of  the  germ 
nuclei,  and  that  when  nuclear  material  from  only  one  parent  is 
functional  the  characters  of  the  so-called  hybrid  are  deter- 
mined thereby. 

We  might  mention  one  further  possible  interpretation  of 
some  of  the  results  opposed  to  this  conclusion,  and  emphasized 
by  Conklin  and  others,  namely,  that  in  some  cases,  at  least,  the 
fundamental  or  general  traits  of  an  organism  may  be  deter- 
mined immediately  by  the  cytoplasmic  structure  of  the  ovum 
alone  or  chiefly,  while  the  nuclei  are  equally  concerned  in  the 
determination  of  the  more  particular  specific  or  individual 
traits,  often  appearing  relatively  late  in  development.  Such 
a  possibility  seems  to  be  indicated  by  many  of  the  facts  of 
germinal  localization  already  described,  and  it  may  be  that  some 
of  the  results  indicated  above,  non-conformable  with  the 
hypothesis  of  nuclear  determination,  point  in  the  same  direc- 
tion. Conklin  writes  (Science,  XXVII,  89-99) :  "  At  the  time  of 
fertilization  the  hereditary  potencies  of  the  two  germ  cells  are 
not  equal,  all  the  early  development,  including  the  polarity, 


306  GENERAL  EMBRYOLOGY 

symmetry,  type  of  cleavage,  and  the  relative  positions  and 
properties  of  future  organs  being  predetermined  in  the  cyto- 
plasm of  the  egg  cell,  while  only  the  differentiations  of  later 
development  are  influenced  by  the  sperm.  In  short,  the  egg 
cytoplasm  fixes  the  type  of  development  and  the  sperm  and  egg 
nuclei  supply  only  the  details."  And  yet  we  should  not  over- 
look this  fact,  of  basic  importance,  that  these  fundamental 
cytoplasmic  differentiations  have  resulted  from  interactions 
between  the  cytoplasm  and  the  nucleus  of  the  oogonial  cell, 
and  that  the  nucleus  of  the  oogonial  cell,  and  egg  cell,  is  itself 
originally  derived  in  equal  parts,  from  paternal  and  maternal 
ancestry.  And  further  many  of  the  conditions  of  polarity, 
symmetry,  and  the  like,  may  in  some  cases  be  determined  or 
altered  by  the  entrance  and  subsequent  activity  of  the  spermat- 
ozoon within  the  ovum. 

Probably  altogether  the  most  striking  evidence  in  support 
of  the  hypothesis  under  consideration  is  to  be  found  in  some 
of  the  recent  work  upon  the  association  of  sex  with  chromosome 
characters.  The  nature  of  this  association  is  so  particular  and 
significant  that  certain  chromosomes  are  actually  regarded  by 
many  as  representing  sex  "  determiners."  This  relation, 
besides  affording  striking  evidence  in  this  connection,  is  of  very 
great  importance  in  itself,  and  we  may  therefore  consider  it  at 
somewhat  greater  length  than  this  connection  alone  would 
justify. 

During  recent  years  many  instances  have  come  to  light,  of  a 
variation  in  the  number  of  chromosomes  in  different  individuals 
of  a  single  species.  With  but  very  few  exceptions  these 
numerical  differences  are  associated  with  difference  in  sex,  and 
when  any  such  difference  exists  it  is  usually  found  that  the  cells 
of  the  female  contain  one  or  more  chromosomes  in  excess  of  the 
number  found  in  the  male.  In  some  species  then,  the  chromo- 
some number  may  be  uneven  in  one  sex,  and  therefore  not  all 
the  chromosomes  are  paired  structures.  In  other  cases  the 
equivalent  diversity  of  the  chromosome  groups  is  indicated  by 
size  differences  between  the  members  of  a  certain  pair. 


DIFFERENTIATION,  HEREDITY,  SEX  307 

Differences  of  these  kinds  are  now  known  in  many  scores  of 
species  of  many  groups,  from  the  lower  worms  to  man.  It  is 
clearly  impossible  to  include  here  any  extended  history  or 
survey  of  this  fascinating  subject  and  we  can  do  little  more 
than  describe  a  typical  instance  or  two,  and  then  mention 
some  comparisons  which  may  throw  some  light,  from  this  point 
of  view,  upon  the  general  interpretation  of  the  chromosome 
problem. 

Since  the  larger  number  of  the  known  instances  of  this  rela- 
tion are  found  among  the  Arthropoda,  particularly  the  Insecta, 
we  may  select  our  first  illustration  from  this  group.  The  num- 
ber of  chromosomes  in  the  somatic  cells  of  the  common  squash 
bug,  Anasa  tristis  (Henking,  Paulmier),  is  twenty-two  in  the 
female,  and  twenty-one  in  the  male.  How  does  this  difference 
come  about? 

For  an  answer  to  this  question  we  must  observe  the  behavior 
of  the  chromosomes  during  the  process  of  maturation  of  the 
germ  cells.  In  the  process  of  oogenesis,  preparatory  to  the  first 
oocyte  division,  synapsis  occurs  normally,  and  eleven  bivalent 
chromosomes  are  formed.  The  succeeding  steps  in  oogenesis 
are  not  unusual  and  the  result  is  the  formation,  in  each  ovum, 
of  a  group  of  eleven  univalent  chromosomes  representing  every 
pair  of  the  original  somatic  or  oogonial  group. 

The  events  of  spermatogenesis  do  not  run  quite  parallel, 
however.  In  the  somatic  and  spermatogonial  cells,  twenty-one 
chromosomes  are  present,  i.e.,  ten  pairs  plus  one,  and  in  the 
division  of  these  cells  every  chromosome  divides  in  the  usual 
way  (Fig.  142).  In  synapsis  the  paired  elements  fuse,  forming 
ten  bivalent  chromosomes,  but  the  odd  chromosome,  or  X- 
chromosome  remains  free,  and  usually  quite  apart  from  the 
other  chromosomes.  This  X-element,  or  idiochromosome,  may 
be  distinct,  even  throughout  the  growth  period  of  the  spermato- 
gonia,  and  during  the  two  spermatocyte  divisions  it  can  be 
identified  in  many  species  as  a  nucleolus-like  body,  indeed 
formerly  it  was  described  as  a  chromatin  nucleolus.  The  behav- 
ior of  this  body  during  the  maturation  divisions  is  entirely  un- 
_usual.  During  the  first  spermatocyte  division  the  idiochro- 


308 


GENERAL  EMBRYOLOGY 


mosome,  although  univalent,  divides  just  as  the  ten  bivalent 
elements  do,  and  eleven  chromosomes  consequently  pass  into 
the  nuclei  of  the  secondary  spermatocytes.  But  during  the 


FIG.  142. — Maturation  during  the  spermatogenesis  of  the  squash-bug,  Anasa 
tristis,  showing  the  behavior  of  the  X-chromosome  or  idiochromosome.  A,  after 
Wilson,  others  after  Paulmier.  A.  Spermatogonium.  Polar  view  of  equatorial 
plate  showing  twenty-one  chromosomes  (ten  pairs,  plus  one).  The  X-chromo- 
some is  not  distinguishable  at  this  time.  B.  Primary  spermatocyte.  Tetrads 
formed.  C.  Equatorial  plate  of  first  spermatocyte  division.  X-chromosome 
divided.  D.  Anaphase  of  same  division.  The  daughter  X-chromosomes  have 
also  diverged.  E.  Equatorial  plate  of  second  spermatocyte  division.  F.  M eta- 
phase  of  same  division.  The  X-chromosome  lies,  undivided,  between  the  two 
groups  of  daughter  chromosomes.  G.  Anaphase  of  same  division.  The  undi- 
vided X-chromosome  has  passed  to  the  upper  pole,  lagging  behind  the  others. 
H.  Telophase  of  same  division.  X-chromosome  still  distinct. 

mitosis  of  these  secondary  spermatocytes  the  idiochromosome 
fails  to  divide  and  passes  as  a  whole  to  one  pole  of  the  spindle 
(Fig.  142,  F,  G,  H).  The  result  is  that  the  nuclei  of  one-half 
of  the  spermatids,  and  therefore  of  one-half  of  the  spermatozoa, 


DIFFERENTIATION,  HEREDITY,  SEX 


309 


contain  eleven  chromosomes,  while  the  other  half  contain  but 
ten,  lacking  the  idiochromosome. 

Since  the  nuclei  of  all  the  ova  contain  eleven  chromosomes 


FIG.  143. — Diagram  illustrating  the  behavior  of  the  X-chromosome  during 
maturation.  The  X-element  is  shown  in  solid  black.  The  essentials  are  the 
same  in  cases  where  X  is  a  multiple  element,  or  where  it  is  paired  with  a  Y-ele- 
ment  (see  text). 

there  are  but  two  possibilities  in  fertilization.     The  egg  with  its 
eleven  chromosomes  may  be  fertilized  by  a  sperm  with  ten, 


310  GENERAL  EMBRYOLOGY 

giving  a  somatic  group  of  twenty-one;  or  the  egg  with  its 
eleven  chromosomes  may  be  fertilized  by  a  sperm  with  eleven, 
giving  a  somatic  group  of  twenty-two.  And  since  there  are 
equal  numbers  of  ten-  and  eleven-chromosome  spermatozoa, 
there  will  be  approximately  equal  numbers  of  zygotes  with 
twenty-one  and  twenty-two  chromosomes.  These  relations 
are  shown  in  diagrammatic  form  in  Figs.  143,  144. 

Since  this  numerical  difference  between  the  somatic  chromo- 
some groups  is  constantly  associated  with  sex-difference,  males 
possessing  twenty-one,  females  twenty-two  chromosomes,  it 
may  be  said  that  the  presence  of  the  idiochromosome  is  in 

Mature  ova.     All  Spermatozoa, 

of  one  class  Two  classes 


Male 


FIG.  144. — Diagram  of  the  relations  of  chromosome  number  and  sex  during 
fertilization  in  Anasa.  The  essentials  are  the  same  in  cases  where  X  is  a  multiple 
element,  or  where  it  is  paired  with  a  Y-element. 

some  way  connected  with  the  determination  of  the  female,  or 
the  lack  of  it  the  male,  sex. 

Since  Paulmier's  description  of  these  events  in  Anasa,  in 
1899,  a  great  many  similar  instances  have  come  to  light,  and 
more  recently  quite  a  variety  of  conditions  related  to  this  but 
more  or  less  dissimilar  in  details,  have  become  known.  The 
X-chromosorne  or  idiochromosome  has  been  described  under 
many  different  names  such  as  accessory  chromosome,  allosome, 
heterotropic  chromosome,  heterochromosome,  monosome,  etc.  Such 
an  unequal  distribution  of  the  chromosomes  was  first  observed 
by  Henking  in  1890,  and  in  1902  McClung  described  similar 
processes  in  several  of  the  locusts  and  grasshoppers  (Orthop- 
tera),  and  first  suggested  the  possible  relation  between  this 


DIFFERENTIATION,  HEREDITY,  SEX  311 

"accessory  chromosome"  and  the  determination  of  sex.  The 
work  of  Wilson,  Stevens,  Montgomery,  Payne,  Guyer,  Morgan, 
and  many  others,  has  made  known  the  presence  of  these  elements 
in  a  whole  host  of  Insects,  including  most  of  the  orders,  in 
Myriopods,  Arachnids,  and  Copepods.  Among  the  lower 
forms,  Nematodes  (Boveri,  Edwards,  Boring,  Gulick),  Sagitta 
(Stevens),  and  Echinoderms  (Baltzer)  are  now  known  to  pos- 
sess idiochromosomes.  And  more  recently  some  of  the  Chor- 
dates  have  been  added  to  the  ever  increasing  list,  for  idiochro- 
mosomes have  been  described  in  the  common  fowl,  guinea  fowl, 
and  rat  (Guyer),  the  guinea  pig  (Stevens)  and  even  in  man, 
where  Guyer  has  reported  two  idiochromosomes,  half  the  sperm 
containing  twelve,  and  half  ten,  chromosomes;  the  number  of 
chromosomes  in  the  human  somatic  cells  is,  therefore,  twenty- 
two  in  the  male,  and  twrenty-four  in  the  female. 

Not  all  of  the  forms  included  in  the  above  list  exhibit  this 
phenomenon  as  simply  as  it  occurs  in  the  case  of  Anasa;  we  may 
mention  two  general  modifications  of  this  typical  condition. 
In  many  Coleoptera,  Diptera,  and  Hemiptera,  the  idiochromo- 
some  is  not  strictly  an  unpaired  element  for  during  the  sperma- 
tocyte  divisions,  and  in  the  spermatids,  it  is  paired  with  a  very 
small  chromosome  called  the  Y-element;  together  with  X  this 
makes  up  an  XY  bivalent  chromosome  which  behaves  like  any 
bivalent  chromosome  in  the  preliminaries  to  the  first  spermato- 
cyte  division  (Fig.  145).  In  the  spermatozoa,  therefore,  half 
the  nuclei  contain  the  large  idiochromosome  (X),  and  half  the 
small  one  (Y).  The  relation  to  sex  is  what  might  be  expected, 
namely,  the  females  contain  the  large  X,  the  males  the  small  Y. 
In  Metapodius,  one  of  the  Orthoptera,  this  small  Y-element  may 
be  either  present  or  absent  apparently,  although  it  is  possible 
that  it  may  be  present  and  fused  with  another  chromosome 
when  it  is  said  to  be  absent. 

In  Ascaris  megalocephala  it  seems  clear  that  a  small  X- 
element,  no  Y-element  being  present,  may  thus  appear  either  as 
a  separate  body,  or  fused  with  one  of  the  other  chromosomes 
(Boring,  Boveri,  Edwards).  Such  an  attachment  of  the  idio- 
chromosome to  a  certain  one  of  the  ordinary  chromosomes  is 


312 


GENERAL  EMBRYOLOGY 


Y 
X 


Protenor 
Anasa 


Syromasfas 
Homo 


As  car  is 
[umbricoides 


fr     i    i     t 

Ncznra         Eitschwtu.t       Nezara         ThyflK 
Yiricfala        Coenus  /lifaris 


catceata 


Y 
X 


Prionitfus     Getas  tocoris       Acholla 
5inea. 


nosa 

FIG.  145. — Diagrams  illustrating  some  of  the  variations  in  the  X-  and  Y- 
chromosomes.  From  Wilson.  X,  either  as  a  simple  or  as  a  multiple  element, 
may  or  may  not  be  paired  with  a  Y-chromosome. 


L 


A 

t 


e 


Fio.  146. — Compound  chromosome-groups,  formed  by  the  union  of  the  X- 
chromosome  with  other  chromosomes,  in  the  Orthoptera.  From  Wilson,  a,  b, 
after  de  Sinety,  others  after  McClung.  a.  Triad  group,  first  spermatocyte 
division  of  Leptynia,  metaphase.  6.  Division  of  similar  triad  in  Dixippus. 
c.  Triad  group  formed  by  union  of  the  X-chromosome  with  one  of  the  bivalent 
chromosomes,  first  spermatocyte  prophase,  Hesperotettix.  d.  The  same  element 
from  a  metaphase  group,  e.  The  same  element  in  the  ensuing  interkinesis. 
/.  The  compound  element  of  Mermiria,  from  a  first  spermatocyte  prophase. 
g.  The  same  element  in  the  metaphase  (now,  according  to  McClung,  united  to 
a  second  bivalent  chromosome,  to  form  a  pentad  element),  h.  The  same  element 
after  its  division,  in  the  ensuing  telophase. 


DIFFERENTIATION,  HEREDITY,  SEX  313 

well  known  as  the  constant  relation  in  some  Insects  (Sinety, 
McClung),  and  among  these  forms  various  degrees  of  the  inti- 
macy of  the  association  occur  (Fig.  146). 

Several  stages  can  be  found  in  the  gradual  increase  hi  the 
relative  size  of  the  Y-element,  until  in  such  forms  as  Nezara 
hilaris,  one  of  the  Hemiptera-Heteroptera  described  by  Wilson, 
X  and  Y  are  nearly  equal  (Fig.  145),  and  finally  in  some  of  the 
Lepidoptera  and  other  forms,  X  and  Y  are  quite  equal  and 
indistinguishable  from  one  another,  although  the  XY  pan*  may 
be  distinguished  from  the  other  chromosomes  by  staining  prop- 
erties and  behavior. 

We  thus  reach  through  gradual  transitions  a  condition  where 
the  spermatozoa  are  no  longer  dimorphic  with  respect  to  chro- 
mosome content.  The  conditions  of  such  a  series  suggest,  how- 
ever, the  possibility  that  spermatozoa  that  visibly  appear 
morphologically  alike,  may  after  all  be  physiologically  dimor- 
phic as  regards  chromosome  characters;  such  an  assumption 
must  of  course  be  made  with  respect  to  traits  other  than  sex, 
which  are  inherited  in  an  alternative  fashion. 

Another  series  of  modifications  of  the  Anosa-type  is  illus- 
trated by  various  genera  of  Hemiptera,  where  the  X-chromo- 
some  is  represented  by  more  than  a  single  element.  Such  a 
series  has  been  described  by  Payne,  and  is  readily  derived 
from  the  condition  in  such  a  form  as  Euschistus  (Fig.  145), 
with  unequal  X-  and  Y-element s.  Thus  in  Fitchia  and  several 
others,  Y  is  a  fairly  large  chromosome  while  X  is  represented 
by  two  somewhat  smaller  chromosomes;  hi  Prionidus,  Sinea,  etc. 
there  are  three  X-elements  to  one  Y;  in  Gelastocoris  there  are 
four  X  and  one  Y,  and  in  Acholla  five  X  and  one  Y.  In  still 
another  series  (Fig.  145)  Y  is  entirely  absent  and  X  is  repre- 
sented by  several  chromosomes — two  in  Phylloxera  (Morgan), 
Syromastes  (WHson),  Agalena  (Wallace),  and  man  (Guyer), 
five  in  Ascaris  lumbricoides  according  to  Edwards. 

In  all  of  these  cases  where  X  is  a  multiple  element, 
different  species  show  greatly  varying  relations  among  the 
members  of  the  X-group;  they  may  be  approximately  equal 
in  size  or  very  unequal,  but  it  is  important  that  the  size 


314 


GENERAL  EMBRYOLOGY 


relations,    whatever   they  are,   are  constant   within  a  given 
species. 

One  further  general  condition  of  the  idiochromosomes  must 
be  noted.  In  all  of  the  instances  mentioned  above  the  sper- 
matozoa are  the  dimorphic  gametes,  i.e.,  the  males  are  "diga- 
metic" (Wilson's  term).  In  a  few  species,  however,  it  is  the 
female  that  is  digametic,  and  while  the  spermatozoa  are  all 
alike,  the  ova  are  of  two  classes  with  respect  to  certain  modified 
chromosomes  which  may  properly  be  regarded  as  homologous 
with  the  X-  and  Y-elements  of  the  spermatozoa.  Thus  in  two 
of  the  sea-urchins,  Strongylocenlrotus,  and  Echinus,  Baltzer 


FIG.  147. — Chromosomes  in  the  sea-urchin,  Strongylocentrotus  lividus.  After 
Baltzer.  X  2610.  A.  First  cleavage  spindle  (reconstructed  from  two  draw- 
ings). No  small  hooks  present.  B.  First  cleavage  spindle  with  small  hooks. 
The  modified  chromosomes,  both  long  and  short  hooks,  are  shown  in  solid  black. 

finds  the  chromosome  groups  of  the  spermatids  all  alike,  with 
eighteen  components  uniformly  differentiated;  the  nuclei  of 
some  of  the  ova,  however,  are  characterized  by  the  presence 
of  a  single  modified  component  which  is  absent  from  the 
remainder  (Fig.  147).  This  obviously  corresponds  with  the 
X-element  of  the  dimorphic  sperm.  There  is  also  very  good 
reason  for  believing  the  female  digametic  in  some  of  the 
Lepidoptera;  this  is  of  considerable  interest  in  view  of  the  fact 
that  repeated  investigation  has  thus  far  failed  to  disclose 
idiochromosomes  in  the  spermatocytes  of  these  Insects. 


DIFFERENTIATION,  HEREDITY,  SEX  315 

The  idea  that  the  relation  between  the  chromosomes  and 
sex  characters  parallels  that  between  the  chromosomes  and 
any  other  traits  involves  the  conclusion  that  sex  is  a  character, 
or  group  of  characters,  inherited  in  the  same  way  that  other 
bodily  traits  are.  And  this  conclusion  may  now  be  accepted. 
Indeed  there  are  in  the  field  several  hypotheses  as  to  the  precise 
statement  of  a  Mendelian  formula  according  to  which  sex  is 
inherited,  and  while  no  one  of  them  has  a  preponderance  of 
evidence  in  its  favor,  the  fundamental  fact  of  sex  heredity  is 
clear. 

There  are  extant  scores  of  hypotheses  regarding  the  factors 
and  processes  involved  hi  sex  determination,  depending  upon 
the  action  of  conditions  outside  of  the  germ  itself.  These  must 
be  abandoned  when  the  facts  now  known  to  be  true  of  the 
germinal  structure  of  a  comparatively  limited  number  of 
species,  gain  a  wider  applicability.  For  the  sex  of  an  organism, 
as  well  as  other  fundamental  characters,  appears  to  be  already 
determined  hi  the  zygote,  and  all  that  external  conditions  can 
do  toward  determining  sex  is  to  alter  sex  ratios  by  affecting 
differentially  (selectively)  the  gametes  or  immature  organisms 
of  a  certain  sex. 

There  is  some  evidence  of  other  kinds  that  sex  is  determined 
in  the  gamete  and  not  by  external  conditions.  In  certain 
cases  a  single  egg  indirectly  gives  rise  to  a  number  of  embryos 
or  larva?  (multiple  embryo  formation)  which  are  all  of  one  sex, 
either  male  or  female.  Silvestri  describes  such  a  case  in  the 
development  of  a  Hymenopter,  Litomastix,  parasitic  in  the 
larva  of  a  Lepidopter,  Plusia,  where  as  many  as  one  thousand 
embryos,  all  of  one  sex,  are  thus  formed.  And  there  is  good 
reason  for  believing  that  the  embryos  of  one  of  the  armadillos, 
described  by  Newman  and  Patterson,  are  all  derived  from  a 
single  ovum,  and  these  are  always  of  one  sex  only,  either  male 
or  female.  There  is  also  the  familiar  example  of  the  bees 
(Dzierzon),  where  unfertilized  eggs  develop  parthenogenetically 
into  males  (drones),  while  the  fertilized  ova  produce  females 
(queen  and  workers) ;  the  same  thing  is  apparently  true  of  most 
ants.  And  in  one  of  the  rotifers,  Hydatina,  a  certain  kind  of 


316  GENERAL  EMBRYOLOGY 

female  produces  eggs  which  if  fertilized  produce  females,  if 
unfertilized  produce  males. 

One  further  point  is  to  be  mentioned  in  connection  with 
the  general  hypothesis  of  nuclear  determination.  This  is  in 
connection  with  that  curious  form  of  Mendelian  heredity 
known  as  " sex-limited"  heredity,  where  certain  characters 
are  exhibited  by  the  individuals  of  one  sex  only,  although 
transmitted  by  the  individuals  of  the  other  sex  without  being 
exhibited  by  them.  Such  a  form  of  heredity  can  readily  be 
explained  upon  the  chromosome  hypothesis,  upon  the  simple 
assumption  of  a  close  relation  between  the  " determiner"  for 
the  sex-limited  character  and  the  sex-determining  element. 

Any  further  consideration  of  the  problems  of  sex  determina- 
tion and  heredity  would  lead  us  too  far  afield;  more  extended 
treatment  must  be  had  from  other  sources.  In  conclusion 
then,  it  is  hardly  necessary  to  point  out  that  the  constancy  of 
the  form  and  of  the  complicated  behavior  of  these  idiochromo- 
somes  affords  very  striking  confirmation  of  the  hypothesis  of 
the  specificity  and  genetic  continuity  of  the  chromosomes. 
While  it  is  possible  that  their  form  and  behavior  are  determined 
by  underlying  conditions,  such  conditions  cannot  be  directly 
observed  and  can  only  be  postulated.  Taken  in  connection 
with  the  facts  mentioned  in  Chapter  II,  and  with  the  results 
drawn  from  the  development  of  dispermic  eggs,  and  from 
hybridization,  they  amount  to  practical  demonstration  of 
some  form  of  chromosomal  specificity  in  development. 

As  to  the  question  whether  the  idiochromosomes  are  in 
particular  the  sex  determinants,  several  views  may  be  held  in 
the  absence  of  conclusive  experimental  demonstration  of  the 
precise  relation.  It  has  been  held  in  some  quarters  that  sex 
is  determined  by  the  relative  amount  of  chromatin  received 
into  the  nucleus  of  the  zygote,  irrespective  of  its  content  in 
certain  chromosomal  elements.  This  is  hardly  tenable  how- 
ever in  view  of  many  contradictory  conditions.  Others  have 
suggested  that  the  dimorphism  of  the  gametes  is  merely  asso- 
ciated with  other  more  fundamental  diversities,  and  that  sex 
differentiation  and  gamete  differentiation  are  related  only  be- 


DIFFERENTIATION,  HEREDITY,  SEX  317 

cause  both  are  related  to  some  primary  differentiation.  Still 
others  hold  to  the  idea  that  the  idiochromosomes  actually 
determine  by  their  presence  or  absence,  the  nature  of  the 
reactions  of  development,  so  that  finally  organisms  with  female 
or  male  characteristics  are  formed.  The  most  adequately  justi- 
fied and  most  conservative  view  seems  to  be  that  the  nature 
of  the  interrelations  of  the  components  of  the  whole  chromo- 
some group,  among  themselves  and  to  the  cytoplasm,  is  modi- 
fied by  the  presence  or  absence  of  certain  elements  so  that  in 
one  case  the  primary  and  secondary  female  characters  develop, 
in  the  other  case  the  male  characters. 

Returning  now  to  the  general  subject  of  this  chapter,  namely, 
the  factors  determining  the  course  of  development  and  the 
process  of  heredity,  we  come  to  another  extremely  important 
subject.  We  have  thus  far  emphasized  the  importance  of  the 
internal  factors  of  development.  But  we  have  denned  develop- 
ment as  a  series  of  reactions  between  internal  and  external 
factors.  The  omission  hitherto  of  specific  reference  to  the 
external  conditions  of  development  is  not  because  these  are  of 
lesser  general  importance.  Alterations  in  the  conditions  of 
gravity,  pressure,  temperature,  light,  moisture,  and  chemical 
composition  of  the  surrounding  medium  may,  each  or  all  affect 
the  course  of  development,  either  in  a  general  or  in  a  specific 
way.  A  great  deal  is  known  of  the  results  of  modifying  such 
conditions  and  a  rather  full  discussion  of  these  effects  would  be 
in  order,  were  the  results  susceptible  of  more  definite  and  more 
uniformly  applicable  statement.  For  normal  development, 
normal  environing  conditions  are  necessary.  However,  slight 
variations  in  external  conditions  rarely  produce  effects  com- 
parable with  those  following  slight  variations  in  the  internal 
conditions.  That  is  to  say,  slight  variations  in  external  condi- 
tions are  "normal."  When  the  modification  of  external  condi- 
tions is  sufficiently  marked  to  produce  visible  effects  upon 
development,  these  are  frequently  so  marked  as  to  be  regarded 
as  distinct  abnormalities,  and  the  organism  so  affected  is  rarely 
able  to  complete  its  development  to  maturity. 


318  GENERAL  EMBRYOLOGY 

The  natural  environment  ordinarily  varies  within  rather 
narrow  limits,  frequently  on  account  of  the  ovipositing  habits 
of  the  adult,  and  changes  within  these  limits  rarely  affect  the 
course  of  development,  so  that  for  the  subjects  of  heredity  and 
differentiation  we  should  inquire  here  only  into  the  question  to 
what  extent  external  conditions  are  necessary  factors  in  carry- 
ing on  the  life  of  the  organism  as  it  exists  in  the  form  of  an 
egg  or  embryo.  The  particulars  of  development  and  heredity 
are  referable  to  internal  characteristics  which  determine  the 
specific  or  individual  quality  of  the  reactions  between  organism 
and  environment. 

We  may  proceed,  therefore,  to  mention  a  few  illustrations  of 
the  effects  of  alterations  in  external  conditions  of  development, 
not  attempting  to  do  more  than  to  suggest  the  nature  of  the 
work  accomplished  in  this  field;  an  adequate  survey  falls  outside 
the  scope  of  such  a  text  as  this.  (For  a  convenient  summary, 
see,e.^.,  Jenkinson,  "Experimental  Embryology/7  Oxford,  1909.) 

More  is  known  regarding  the  effects  upon  development,  of 
chemical  substances  than  of  other  conditions.  While  a  few 
forms,  such  as  the  minnow,  Fundulus,  are  able  to  develop 
normally  in  media  so  widely  unlike,  physically  and  chemically, 
as  sea  water  and  distilled  water,  this  and  other  forms  show 
specific  effects  of  the  presence  or  absence  of  certain  salts  alone. 
Thus  in  Fundulus  Stockard  has  shown  that  the  presence  of  cer- 
tain amounts  of  magnesium  salts  brings  about  the  fusion  of  the 
optic  vesicle  regions,  so  that  one-eyed  monsters  develop,  appar- 
ently normal  in  other  respects  (Fig.  148).  The  eggs  and  em- 
bryos of  the  Echinoderms  offer  many  striking  facts  in  this 
connection.  We  have  already  noted  that  the  alkalinity  of  the 
sea  water  may  determine  the  appearance  of  paternal  or  mater- 
nal characters  in  hybrid  Echinoderm  larvae.  Herbst  and  others 
have  shown  that  the  absence  of  potassium  salts  is  fatal  or  very 
harmful  to  Echinoderm  larvae,  apparently  on  account  of  the 
resulting  diminution  in  the  process  of  water  absorption;  the 
absence  of  calcium  causes  a  tendency  for  the  blastomeres  to  fall 
apart;  magnesium  and  the  sulphates  are  necessary  for  the  nor- 
mal differentiation  of  the  alimentary  tract;  the  production  of 


DIFFERENTIATION,  HEREDITY,  SEX 


319 


ciliary  movement  depends  upon  the  presence  of  magnesium, 
and  an  excess  of  calcium  results  in  the  hypertrophy  of  the  cilia; 
sulphates  are  necessary  also  for  the  establishment  of  the  funda- 
mental structure  of  the  embryo  and  for  the  formation  of 
pigment;  magnesium,  sulphates,  and  calcium  carbonate  are 


B 


•••-••»<m/ 


FIG.  148. — Effects  of  magnesium  chloride  upon  the  development  of  the  Teleost, 
Fundulus.  From  Stockard.  A.  Normal  fish,  eight  days  after  hatching. 
M,  mouth.  B,  C.  Two  views  of  fish,  showing  the  fusion  of  the  optic  vesicles 
as  the  result  of  treatment  with  MgCh. 

necessary  for  the  development  of  a  normal  skeleton  (Fig.  149). 

Certain  optima  exist  for  moisture,  density,  pressure,  light  and 
temperature;  in  development  as  in  later  life,  deviations  from 
the  optimum  condition,  in  either  direction,  affect  the  rate  of 
development  rather  than  its  character.  The  direction  of 
gravity  takes  an  essential  part  in  determining  normal  develop- 
ment in  a  few  cases,  but  ordinarily  development  is  independent 
of  this  factor. 

In  general  all  of  these  conditions  are  involved  not  so  much  in 
the  regulation  of  development  in  specific  and  particular  direc- 
tions, as  in  determining  whether  it  shall  proceed  at  all  or  not. 
Modifications  of  development  produced  by  effective  variations 


320 


GENERAL  EMBRYOLOGY 


in  these  conditions  are  often  so  extreme  that  the  phenomena  of 
heredity  are  scarcely  apparent  and  usually  the  modified  organ- 
ism does  not  come  to  maturity. 

We  may  now  attempt  to  summarize  a  conservative  concep- 
tion of  the  relation  of  the  structure  of  the  germ  cells  to  the  pro- 
cesses of  development  and  heredity.  The  zygote  is  an  organ- 
ism, morphologically  and  physiologically  specific.  It  possesses 


a 


h 


FIG.  149. — Effects  of  chemical  alteration  of  the  surrounding  medium,  upon  the 
development  of  the  sea-urchin.  From  Jenkinson.  a.  Without  OH;  ciliated 
solid  blastula  of  Sphoer  echinus,  b.  KOH  has  been  added,  c.  Normal  blastula 
of  Sphcer echinus,  d.  Blastula  in  a  K-free  medium,  e.  Reared  in  K-free  medium 
and  replaced  in  normal  sea- water  (Sphcerechinus) .  f.  Sphcerechinus  larva  from  a 
medium  devoid  of  Mg.  g.  Echinus  pluteus  with  three-parted  gut,  mouth,  and 
ccelomic  sacs,  but  neither  skeleton  nor  arms;  reared  without  CaCOs  or  CaSO4. 
h.  Normal  pluteus  of  Echinus. 

polarity,  symmetry,  various  forms  of  differentiated  substance, 
even  organs,  composed  of  subsidiary  elements  and  capable  of 
performing  definite  and  highly  varied  and  specialized  functions. 
This  organism  in  its  parts  and  as  a  whole  does  certain  things, 
makes  certain  reactions,  in  a  word,  develops.  The  quality  of 
the  developmental  reactions  is  determined  primarily  by  the 


DIFFERENTIATION,  HEREDITY,  SEX  321 

conditions  within  the  organism  itself,  and  as  it  reacts,  as  the 
organism  develops  step  by  step,  these  internal  conditions  rapidly 
change.  These  reactions  on  the  part  of  the  organism  fall  into 
two  groups.  (1)  Reactions  between  the  organism  (i.e.,  cyto- 
plasm and  nucleus,  whether  the  organism  consists  of  one  cell  or 
many)  and  its  environing  stimuli.  (2)  Reactions  between  the 
nucleus  and  cytoplasm  of  each  cell.  The  idea  of  reaction  must 
involve  two  factors,  but  while  equally  necessary  for  reaction, 
they  are  not  necessarily  of  equal  value  in  determining  or  con- 
trolling the  quality  of  the  reaction.  A  great  many  organisms 
react  to  light;  but  the  quality  of  the  reaction  is  determined 
primarily  by  the  organism. 

The  whole  structure  of  the  cytoplasm  may  play  a  large  part 
in  determining  the  quality  of  the  reactions  of  the  egg,  but  this 
cytoplasmic  structure  is  itself  the  result  of  a  series  of  interac- 
tions between  cytoplasm  and  nucleus,  and  the  action  of  the 
latter  is  of  primary  importance  in  affecting  the  quality  of  the 
result.  Going  one  step  further,  what  the  nucleus  does  is  deter- 
mined by  its  structure,  and  this  is  also  the  result  of  interactions 
of  its  parts  with  one  another,  and  with  the  cytoplasm,  which  is 
its  environment;  and  here  again  certain  elements  of  the  nucleus, 
namely,  the  chromosomes,  seem  to  be  of  primary  importance 
in  determining  the  quality  of  the  interaction.  The  most  impor- 
tant of  the  concrete,  visible  organs  of  the  nucleus  are  the  chro- 
mosomes. And  when  we  attempt  to  analyze  the  behavior  of 
these  components  we  are  met  by  the  same  problem — what 
determines  the  structure  and  behavior  of  these?  Two  answers 
have  been  offered.  First,  that  here  we  reach  the  limit  of  analy- 
sis, that  the  chromosomes  are  autonomic,  self-perpetuating, 
self-regulating  bodies,  whose  morphology  and  behavior  are  the 
determining  factors  in  all  that  happens  in  the  life  of  the  organ- 
ism. Second,  that  the  chromosomes  are  themselves  made  up 
of  still  more  fundamental  units,  the  chromatin  granules;  that 
these  are  the  autonomic,  self- perpetuating,  finally  determina- 
tive units  in  development,  and  chromosome  structure  is  the 
result  of  the  primary  activity  of  these  bodies. 

Logically  there  is  no  reason  why  we  must  stop  with  the  chro- 


322  GENERAL  EMBRYOLOGY 

matin  granules.  And  history,  enumerating  germ  layers,  cleav- 
age cells,  cytoplasmic  organ-forming  substances,  chromosomes, 
and  chromioles  (chromatin  granules),  warns  us  against  the  idea 
that  we  must  seek  or  hope  to  find  ultimate  particles,  concrete, 
definable,  and  representatively  determinative  in  function.  In 
fact  a  few  students  of  the  problem  frankly  declare  their  belief 
that  the  idea  of  any  sort  of  representative  particle  mechanism 
is  futile,  that  the  regulation  of  the  processes  of  development 
and  heredity  depends  upon  interrelations  which  are  not  sus- 
ceptible of  interpretation  in  terms  of  any  material  basis. 

Scientifically,  however,  we  can  to-day  go  no  further  back 
than  the  chromosomes,  for  here  we  find  the  most  fundamental 
units  whose  actual  behavior  can  be  correlated  with  the  facts 
of  development.  To  say  scientifically,  that  the  chromosomes 
are  (to-day  known  to  be)  the  determining  elements  in  develop- 
ment and  heredity,  is  not  to  deny  the  existence  of  other  bodies 
or  conditions  which  may  determine  the  existence  and  qualities 
of  the  chromosomes.  Granules  we  know,  but  of  their  behavior 
we  know  little,  and  this  little  cannot  at  present  be  correlated 
with  the  facts  of  development.  Of  the  real  existence  of  elements 
underlying  the  granules  we  know  nothing  whatever.  Assump- 
tion of  the  reality  of  such  bodies  or  conditions  may  be  a  logical 
necessity,  but  to-day  it  carries  us  beyond  the  boundaries  of 
observed  fact. 

To  repeat  a  statement  made  on  an  earlier  page,  if  the  exist- 
ence and  activity  of  the  chromosomes  can  be  shown  to  be  a 
necessary  link  in  the  processes  of  development  and  heredity, 
and  if  these  can  be  shown  to  be  the  simplest  and  most  nearly 
primary  factors  whose  behavior  can  be  correlated  with  these 
processes,  then  we  shall  be  justified  in  saying  that  the  chromo- 
somes are  to-day  the  determining  factors  in  development  and 
heredity. 

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DIFFERENTIATION,  HEREDITY,  SEX  327 

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328  GENERAL  EMBRYOLOGY 

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CHAPTER  VIII 
THE  BLASTULA,  GASTRULA,  AND  GERM  LAYERS 

MORPHOGENETIC  PROCESSES 

Ix  this  chapter  we  shall  endeavor  to  summarize  the  more 
general  processes  of  early  development  which  lead  to  the 
formation,  out  of  the  group  of  cleavage  cells,  of  an  embryo 
possessing  the  beginnings  of  the  chief  organs  or  systems.  This 
will  carry  us  from  the  formation  of  the  blastula,  through  the 
important  events  of  gastrulation  and  germ  layer  formation, 
and  the  varied  processes  by  which  tissue  and  organ  rudiments 
are  set  apart  and  differentiated. 

We  shall  give  particular  attention,  indeed  practically  shall 
limit  ourselves,  to  a  descriptive  morphological  account  of 
these  events.  This  is  done,  not  as  a  matter  of  choice,  but 
because  the  experimental  results  of  the  functional  analysis 
of  these  processes  are  so  fragmentary  and  so  scattered,  that  the 
attempt  at  their  summary,  in  a  text  of  this  character,  seems 
unwise.  This  is  partly  because  the  efforts  to  analyze  these 
processes  experimentally  have  been  delayed  until  the  similar 
problems  of  cell  organization  and  cleavage  should  have  been 
more  satisfactorily  solved.  These  topics  we  have  already 
considered.  We  should  say,  however,  that  what  has  already  been 
accomplished  in  the  way  of  describing  these  later  phenomena 
of  the  mechanics  of  development  (Entwicklungsmechanik, 
Roux)  bears  out  the  general  conclusions  indicated  by  the  earlier 
processes,  namely,  that  while  the  action  of  external  conditions 
as  stimuli  is  essential,  their  place  normally  is  chiefly  that  of 
affording  the  general  conditions  of  life  and  development.  The 
actual  quality  and  the  really  significant  details  of  the  later, 
as  of  the  earlier  phenomena  of  development  and  differentia- 
tion, depend  primarily  upon  internal  conditions  and  relations. 

329 


330  GENERAL  EMBRYOLOGY 

And  further,  there  is  little  reason  for  supposing  that  here  too 
the  essential  determinative  conditions  are  other  than  nuclear, 
although  from  the  nature  of  the  case,  the  evidence  must  be 
less  direct. 

We  shall  limit  ourselves  in  another  direction  also.  It  is 
obviously  impossible  to  give  a  brief,  and  at  the  same  time  an 
adequate,  account  of  the  extremely  diverse  methods  of  gas- 
trulation,  germ  layer  formation,  etc.,  in  the  Metazoa  as  a  whole. 
We  shall,  therefore,  confine  ourselves  here  largely  to  the  con- 
sideration of  these  events  among  the  Chordata.  This  will 
enable  us  to  give  a  more  adequate  consideration  to  the  topics 
selected. 

We  have  seen  that  while  no  definite  termination  can  be 
placed  to  the  period  of  cleavage,  there  is  rather  general,  though 
arbitrary,  agreement  that  cleavage  may  be  said  to  have  termi- 
nated when  the  blastomeres  become  arranged  as  a  more  or 
less  definite  epithelium  or  layer,  bounding  an  internal  space  of 
some  sort;  and  further  that  this  may,  in  some  cases,  also  be 
marked  by  the  attainment  of  a  certain  nuclear-cytoplasmic 
relation.  The  organism  exhibiting  these  characteristics  is 
known  as  the  blastula.  In  this  stage  the  organism  is  essentially 
a  monodermic  structure,  that  is,  it  is  composed  of  a  single  tissue 
or  layer  of  somewhat  similar  cells.  In  the  simplest  form  of 
blastula  this  layer  is  but  one  cell  thick  (Amphioxus,  Fig.  150,  A), 
but  in  most  of  the  Chordate  blastulas  the  wall  is  many  cells  in 
thickness. 

Like  the  cleavage  pattern,  though  to  a  much  greater  extent, 
the  general  form  of  the  blastula  is  largely  determined  by  the 
amount  of  yolk  or  deutoplasm  contained  in  the  ovum;  and  the 
form  of  the  blastula,  in  turn,  largely  determines  the  form  of  the 
gastrula,  and  the  methods  of  gastrulation  and  germ  layer 
formation.  We  may  for  convenience,  therefore,  distinguish 
three  general  forms  of  blastulas,  although  intergradations  are 
not  infrequent.  When  the  ova  are  nearly  homolecithal,  and 
cleavage  is  total  and  adequal,  as  in  Amphioxus  (Fig.  150,  A), 
the  blastula  is  practically  a  hollow  sphere  (coeloblastula) .  Its 
wall  is  a  simple  epithelium,  one  cell  in  thickness,  and  its  cavity, 


BLASTULA,  GASTRULA,  AND  GERM  LAYERS    331 

the  blastoccel  or  segmentation  cavity,  is  large  and  nearly  central, 
though  not  quite,  for  nearly  always  the  cells  at  the  vegetal  pole 
are  larger  than  those  at  the  animal  pole,  and  the  wall  conse- 
quently thicker  in  the  former  region.  This  form  of  blastula  is 
commonly  regarded  as  primitive,  though  hardly  typical  of  the 
Clior dates  in  general,  for  among  these  it  is  found  only  in 
Amphioxus. 


en 


FIG.  150. — Types  of  Chordate  blastulae.  A.  Amphioxus  (coeloblastula). 
B.  Petromyzon.  After  von  Kupffer.  C.  Notums  (Teleost)  (discoblastula). 
D.  Triton  (Urodele).  After  Greil.  a,  animal  pole;  c,  blastoccel;  p,  periblast; 
v,  vegetal  pole. 

More  usually  the  Chordate  ovum  contains  a  considerable 
amount  of  yolk,  as  in  the  Ganoids  and  Amphibia.  And  here 
the  cleavage,  though  nearly  or  quite  complete  in  most  cases,  is 
decidedly  unequal  and  the  form  of  the  blastula  is  considerably 
modified  in  consequence.  Here  the  wall  of  the  blastula  is 
several  or  many  cells  thick;  the  cells  of  the  animal  pole  are 
quite  small,  while  the  yolk-containing  cells,  of  the  vegetal 
hemisphere,  are  very  large.  The  blastoccel  is  therefore  quite 
eccentric,  displaced  toward  the  animal  pole,  and  usually 
much  reduced  in  size  (Fig.  150,  B,  D). 


332  GENERAL  EMBRYOLOGY 

The  most  highly  modified  type  of  Chordate  blastula  is  found 
in  those  forms  with  extremely  meroblastic,  telolecithal  eggs, 
where  cleavage  is  of  the  discoid  type.  This  condition  is  com- 
mon to  the  Elasmobranchs  (Fig.  158),  the  true  Teleosts  (Fig. 
150,  C),  and  to  the  higher  Craniates — the  Reptiles,  Birds.  In 
reality  this  is,  in  a  modified  way,  characteristic  of  the  Mam- 
mals also,  for  although  the  Mammalian  ovum  is  nearly  alecithal, 
it  is  clearly  derived  from  the  Reptilian  condition,  and  many 
features  of  its  development  show  unmistakably  the  effects  of  a 
large  yolk  content  previously  present,  but  now  lost  in  correla- 
tion with  the  newly  acquired  source  of  nourishment  possessed 
by  the  Mammalian  embryo.  The  result  of  discoid  cleavage  is 
the  formation  of  a  small  mass  of  living  active  cells,  the  blasto- 
derm, or  blastodisc,  or  germ  disc,  lying  upon  the  surface  of  the 
yolk  mass.  The  blastula  instead  of  being  spherical,  has  there- 
fore the  form  of  a  circular  disc,  the  cellular  elements  of  which 
can  really  be  compared,  at  first,  only  with  the  cells  of  the  animal 
pole  of  the  spherical  blastula,  the  unsegmented  yolk  represent- 
ing, in  this  stage,  the  large  cells  of  the  vegetal  pole  of  such  a 
blastula.  In  comparing  these  two  types  of  blastulas  we  may 
imagine  that  the  ordinary  spherical  blastula  has  been  cut  in  two 
horizontally,  through  or  just  above  its  equator,  and  the  animal 
hemisphere  flattened  out,  its  circumference  being  thereby 
somewhat  extended.  This  form  of  blastula  (discoblastula)  is 
several  cells  in  thickness  and  is  usually  separated  from  the 
underlying  yolk  by  a  shallow  space  called  the  sub-germinal 
cavity  which  represents  the  blastoccel  (Fig.  150,  C).  While 
the  yolk  mass  is  usually  by  no  means  wholly  devoid  of  nuclei, 
these  may  or  may  not  be  associated  with  true  cellular  structures, 
and  even  when  present  at  this  stage  these  rarely  give  rise  to 
any  structural  parts  of  the  definitive  embryo  when  this  forms. 
The  Mammalian  blastula  diverges  widely  from  any  of  these 
conditions  and  on  account  of  its  very  special  character  further 
reference  to  it  may  be  omitted  here. 

The  next  important  step  in  development  consists  essentially 
in  the  conversion  of  this  monodermic  blastula  into  a  didermic 
organism,  that  is,  one  in  which  the  cells  are  arranged  in  two, 


BLASTULA,  GASTRULA,  AND  GERM  LAYERS    333 

more  or  less  distinct,  tissues  or  layers.  This  process  is  known 
as  gastrulation,  and  the  didermic  embryo  itself  is  called  a 
gastrula. 

Among  the  lower  forms  the  process  of  gastrulation  often 
remains  a  simple  one,  involving  little  more  than  the  mere 
rearrangement  of  the  cells  of  the  blastula  into  two  nearly 
homogeneous  layers.  But  in  the  higher  forms,  such  as  the 
Chordata,  with  which  we  are  dealing,  the  process  is  greatly 
complicated  by  the  precocious  formation  of  the  rudiments  of 
the  chief  axial  structures  of  the  later  developing  embryo,  as 
well  as  by  the  differentiation  of  a  third  tissue,  or  intermediate 
layer  between  the  other  two.  The  establishment  of  the  rudi- 
ments of  the  axial  notochord  and  central  nervous  system, 
characteristic  structures  of  the  Chordate  embryo,  is  termed 
notogenesis.  These  rudiments  are  formed  out  of  the  substance 
of  the  two  primary  layers  of  the  gastrula.  But  the  formation 
of  a  tissue  between  these  converts  the  didermic  embryo  into  a 
tridermic  organism. 

It  is  possible  to  analyze  the  whole  process  of  development 
during  this  period  into  these  three  subsidiary  processes,  gas- 
trulation, notogenesis,  and  middle  layer  formation,  and  in 
some  instances  they  may  occur  somewhat  separately  and 
successively.  But  usually  there  is  much  overlapping,  and 
the  attempt  to  describe  the  process  of  gastrulation  by  itself 
would  give  a  very  incomplete  and  incorrect  view  of  the  events 
of  this  period.  Consequently  we  shall  describe  all  three  of  these 
processes  together. 

Three  general  types  of  gastrulas  and  modes  of  gastrulation 
may  be  found,  corresponding  with  the  three  types  of  blastulas 
and  ova  mentioned  above.  Again  the  simplest  and  probably 
the  most  primitive,  though  not  the  most  typical,  condition  is 
found  in  Amphioxus.  On  the  posterior  side  of  the  blastula,  in 
the  region  just  between  animal  and  vegetal  hemispheres, 
Cerfontaine  has  described  a  small  group  of  cells  marked  by  a 
tendency  to  rapid  and  continued  multiplication  (Fig.  151,  A). 
This  region  of  active  proliferation  gradually  extends  laterally 
around  the  blastula  and  ultimately  forms  a  nearly  complete 


334  GENERAL  EMBRYOLOGY 

ring,  though  this  is  not  until  the  blastula  has  become  converted 
into  the  gastrula.  This  specialized  group  of  cells  may  be 
termed  the  germ  ring,  for  it  is  evidently  equivalent  to  the 
structure  already  known  by  that  name  in  the  Fishes,  Amphibia, 
and  other  forms.  At  the  time  this  rapid  proliferation  com- 
mences, the  vegetal  hemisphere  becomes  flattened.  The  large 
cells  of  this  region  then  arch  up  slightly  into  the  blastocoel  and 
soon  begin  to  fold,  or  swing,  inward  about  their  postero- ventral 
margin  as  a  relatively  fixed  point  (Fig.  151,  B,  C,  D).  This 
motion  is  made  possible  by  the  rapid  extension  of  a  sheet  of 
cells  which  come  off  from  the  germ  ring  and  which  are  thus 
drawn  in,  to  line  the  inside  of  the  animal  hemisphere.  Without 
going  into  details  here,  we  may  say  that  finally  the  inturning 
of  the  vegetal  cells  becomes  complete  (Fig.  151,  E)  and  the 
blastula  is  converted  into  a  cup-like  structure,  widely  open 
toward  one  side  (the  posterior  or  postero-dorsal). 

The  wall  of  the  organism  is  now  composed  of  two  layers  or 
epithelia,  the  original  blastocoelic  cavity  is  nearly  or  quite 
obliterated,  and  a  new  cavity  is  formed,  lined  by  the  inturned 
cells,  and  widely  open  to  the  outside.  This  structure  is  the 
didermic  gastrula.  The  two  cell-layers  composing  its  wall  are 
the  primary  germ  layers.  The  newly  established  gastrular 
cavity  is  the  archenteron  or  primitive  gut  cavity.  The  super- 
ficial layer  of  cells,  including  the  original  animal  hemisphere  of 
the  blastula  and  also  some  cells  derived  from  the  proliferating 
area,  is  known  as  the  outer  germ  layer,  or  ectoderm,  or  ectoblast, 
or  epiblast;  the  layer  lining  the  archenteron,  partly  the  cells  of 
the  vegetal  hemisphere  of  the  blastula  and  partly  the  cells 
derived  from  the  proliferating  region,  is  known  as  the  inner 
germ  layer,  or  endoderm,  or  entoblast,  or  hypoblast.  The  opening 
of  the  archenteron  to  the  outside  is  the  blastopore;  the  periphery 
of  the  blastopore  is  spoken  of  as  its  margin  or  lip,  and  we  have 
seen  that  it  is  the  region  largely  occupied  by  the  germ  ring: 
it  is  here  that  the  two  primary  germ  layers  are  directly  continuous 
with  one  another. 

The  process  of  infolding,  such  as  is  carried  out  here  by  the 
vegetal  cells  which  come  to  line  the  ventral  region  of  the  archen- 


BLASTULA,  GASTRULA,  AND  GERM  LAYERS    335 


H 


FIG.  151. — Gastrulation  in  Amphioxiis.  After  Cerfontaine.  A.  Blastula 
showing  flattening  of  the  vegetal  pole  and  the  rapid  proliferation  of  cells  in  the 
postero-dorsal  region  (germ  ring).  B.  Flattening  more  pronounced;  mitoses 
in  cells  of  germ  ring.  C.  Commencement  of  the  infolding  (invagination)  of  the 
cells  of  the  vegetal  pole.  D.  Continued  infolding  and  inflection,  or  involution, 
of  ectoderm  cells  in  the  dorsal  lip  of  the  blastopore.  The  blastocrel  becoming 
obliterated  and  the  archenteron  being  established.  E.  Invagination  complete. 
Continued  involution  in  the  dorsal  lip  of  blastopore.  Mitoses  in  germ  ring. 
F.  Constriction  of  blastopore  and  commencement  of  elongation  of  the  gastrula. 
Remnants  of  blastocoel  in  ventral  lip  of  blastopore.  G.  Gastrulation  completed. 
Continued  elongation,  and  narrowing  of  blastopore.  H.  Neurenteric  canal 


336  GENERAL  EMBRYOLOGY 

teron,  is  known  as  imagination,  and  in  some  of  the  lower  forms 
the  endoderm  is  wholly  formed  by  this  process.  The  process 
of  inturning,  such  as  results  in  the  lining  of  the  dorsal  region 
of  the  archenteron  by  the  cells  derived  from  the  margin  of  the 
blastopore,  is  known  as  involution.  In  some  forms  the  endo- 
derm is  largely  formed  by  this  process.  Usually  this  is  accom- 
panied by  the  growth  of  the  margin  of  the  blastopore,  or  germ 
ring,  on  over  and  past  the  involuted  region,  so  that  a  layer  of 
cells  is  continually  being  overgrown,  leading  to  more  extended 
gastrulation;  this  process  of  overgrowth  may  be  termed  epiboly. 
The  blastopore  of  the  Amphioxus  gastrula,  at  first  widely 
open,  soon  closes  rapidly  on  account  of  the  growth  and  epiboly 
of  its  lip.  This  process  leads  also  to  the  rapid  elongation  of  the 
gastrula  (Fig.  151,  F-H). 

Although  we  have  included  involution  and  epiboly  as  gas- 
trulation processes  they  are  here  more  properly  to  be  regarded 
as  processes  leading  to  the  formation  of  the  notochord  and  the 
intermediate  layer.  For  as  the  gastrula  continues  to  elongate 
these  structures  begin  to  differentiate  in  the  anterior  part  of  the 
roof  of  the  archenteron,  from  that  part  of  the  inner  layer  formed 
by  involution  and  epiboly.  Along  the  dorso-lateral  regions  of 
the  archenteron  appear  a  pair  of  folds  out  of  the  archenteron, 
each  containing  a  narrow  groove  (Fig.  152,  B,  C).  These  folds 
are  the  rudiments  of  the  intermediate  layer,  or  mesoderm,  or  meso- 
blast;  and  the  grooves,  known  as  the  enteroccelic  grooves,  are  the 
rudiments  of  the  ccelom,  the  cavity  of  the  mesoderm.  That 
portion  of  the  archenteric  roof  between  the  mesoderm  folds, 
later  becomes  folded  outward  and  forms  the  rudiment  of  the 
notochord.  The  remainder  of  the  archenteric  wall,  namely  the 
ventral  and  ventro-lateral  regions  formed  by  the  invaginated 
portion  of  the  inner  layer,  forms  the  primitive  gut  or  enteron. 
Whether  we  choose  to  call  the  involuted  region  really  endoderm 
or  not,  is  immaterial;  if  we  do,  then  we  must  say  that  the  chorda 

established  by  overgrowth  of  neural  folds.  Continued  mitosis  in  germ  ring, 
a,  animal  pole;  ar,  archenteron;  b,  blastoporal  opening;  ch,  rudiment  of  noto- 
chord; dl,  dorsal  lip  of  blastopore;  ec,  ectoderm;  en,  endoderm;  gr,  germ  ring; 
nc,  neuenteric  canal;  nf,  neural  fold;  np,  neural  plate;  s,  blastocoel  or  segmenta- 
tion cavity;  v,  vegetal  pole;  vl,  ventral  lip  of  blastopore;  //,  second  polar  body. 


BLASTULA,  GASTRULA,  AND  GERM  LAYERS    337 


np  nf 


D 


FIG.  152. — Transverse  sections  through  young  embryos  of  Amphioxus,  showing 
formation  of  nerve  cord,  notochord,  and  mesoderm.  After  Cerfontaine.  A. 
Commencement  of  the  growth  of  the  superficial  ectoderm  (neural  folds)  above 
the  neural  plate  (medullary  plate).  B.  Continued  growth  of  the  ectoderm  over 
the  neural  plate.  Differentiation  of  the  notochord,  and  first  indications  of  meso- 
derm and  enterocoelic  cavities.  C.  Section  through  middle  of  larva  with  two 
somites.  Neural  plate  folding  into  a  tube.  D.  Section  through  first  pair  of 
mesodermal  somites,  now  completely  constricted  off.  E.  Section  through  middle 
of  larva  with  nine  pairs  of  somites.  Neural  plate  folded  into  a  tube.  Notochord 
completely  separated.  In  the  inner  cells  of  the  somites,  muscle  fibrillae  are  form- 
ing (compare  Fig.  153).  ar,  archenteron;  c,  enteroccel;  ch,  notochord;  ec, 
ectoderm;  en,  endoderm;/,  muscle  fibrillae;  g,  gut  cavity;  ra,  mesoderm  (gastral); 
ms,  mesodermal  somite;  nc,  neural  canal;  nf,  neural  fold;  np,  neural  plate  (med- 
ullary plate) ;  nt,  neural  tube. 


338 


GENERAL  EMBRYOLOGY 


and  mesoderm  are  both  derived  from  endoderm  and  the  process 
of  involution  is  to  be  regarded  as  a  true  gastrulation  process. 

But  only  the  lesser  part  of  the  mesoderm  is  formed  in  the  way 
just  described.  This  part  of  the  mesoderm  is  known  as  the 
gastral  or  parachordal,  or  axial  mesoderm.  If  we  trace  the 
mesoderm  folds,  just  described,  posteriorly,  we  can  follow  them 
into  the  region  of  the  germ  ring  or  blastopore  lip  which  has  now 

become  considerably  thickened  on  ac- 
count of  its  contraction,  and  consists  of 
a  more  or  less  undifferentiated  mass  of 
cells.  This  mass  now  passes  almost  en- 
tirely around  the  blastopore,  laterally 
and  toward  the  ventral  side.  The 
rapid  proliferation  of  the  cells  of  the 
germ  ring  has  early  led  to  the  disap- 
pearance of  the  original  simple  epithe- 
lial arrangement  of  its  cells  (Fig.  153), 
but  as  it  moves  backward,  with  the 
elongation  of  the  gastrula,  it  leaves 
behind  it  (i.e.,  anteriorly  from  it)  its  cell 
products,  which  rapidly  become  differ- 
entiated into  certain  layers.  On  the 

through  larva  of  Amphioxus,  surface  of  the  embryo  a  layer  is  left 
^STth'rS"  which  is  directly  continuous  with  the 
notochord  and  somites,  ectoderm  of  the  original  gastrula  de- 

After     Cerfontaine.     a,       .        ,„  ,  .        *    ,          « 

archenteron  •  e,  enterocoai  •    rived  irom  the  animal  hemisphere  of 

the.  blastula'  Another  laygr  is  differ- 
entiated  lining  the  archenteron  and 
continuous  with  the  layer  already  there,  and  consisting  of  a  ven- 
tral region  continuous  with  the  invaginated  layer  or  true  gut 
endoderm,  a  dorsal  region  continuous  with  the  involuted  layer 
forming  the  rudiment  of  the  notochord,  while  dorso-laterally, 
between  these  two  regions,  the  inner  sheet  is  continuous  with 
the  mesodermal  rudiments  described  above.  In  other  words, 
out  of  the  germ  ring  there  are  gradually  differentiated,  true  cover- 
ing ectoderm,  gut  endoderm,  chordal  cells,  and  mesoderm.  The 
mesoderm  formed  in  this  way,  directly  from  the  germ  ring,  is 


FIG.  153.  —  Frontal  section 


BLASTULA,  GASTRULA,  AND  GERM  LAYERS    339 

called  peristomial  mesoderm,  to  distinguish  it  from  the  gastral 
mesoderm  formed  in  connection  with  the  enteroccelic  grooves. 
Very  soon,  as  we  have  seen,  the  region  which  gives  rise  to  the 
peristomial  mesoderm  comes  to  extend  nearly  to  the  ventral  side 
of  the  blast opore  region.  The  coelomic  spaces  of  this  peristomial 
mesoderm  are  not  formed  as  derivations  of  the  archenteron;  they 
result  from  rearrangements  of  the  mesodermal  cells,  and  are  en- 
tirely independent  of  the  enteroccelic  portions  of  the  ccelom  in 
their  origin,  although  the  two  become  continuous  later  (Fig. 
153).  These  two  forms  of  mesoderm  are  directly  continuous 
with  one  another  and  have  indeed  a  common  primary  origin,  the 
germ  ring  or  margin  of  the  blastopore.  If  we  recognize  the 
essential  difference  between  them  as  that  of  time  of  formation, 
the  altered  circumstances  surrounding  the  formation  of  each 
due  to  this  time  difference,  become  of  secondary  importance  as 
regards  our  real  conception  of  the  mesoderm  and  its  relations 
to  the  other  germ  layers.  Thus  the  relation  of  both  chorda 
and  mesoderm  proper  to  the  cells  of  the  monodermic  blastula 
is  the  same  as  that  of  the  endoderm  proper. 

Stated  in  a  word  then,  the  gastrulation  of  Amphioxus  is  a 
combination  of  invagination  and  involution,  accompanied  by 
epiboly,  and  the  processes  of  notogenesis  and  mesoderm  forma- 
tion are  intimately  bound  up  with  the  formation  of  the  inner 
layer. 

Having  become  familiar  now  with  the  general  ideas  of  gastru- 
lation and  the  terminology  of  the  process  we  may  consider  the 
remaining  forms  of  this  process  in  the  Chordates  much  more 
briefly. 

Our  second  type  of  gastrulation,  as  it  occurs  in  the  Amphibia 
and  Ganoid  Fishes,  may  be  easily  understood  by  comparison 
with  the  preceding.  The  chief  differences  result  from  the  accu- 
mulation of  yolk  in  the  vegetal  pole  of  the  ovum  and  blastula, 
and  the  consequent  comparative  inertness  of  this  region.  That 
is,  the  chief  modifications  of  the  typical  process  of  gastrulation 
appear  in  respect  to  the  behavior  of  the  lower  pole,  destined  to 
form  the  inner  layer  of  the  gastrula. 

In  the  ^Amphibian  blastula,  the  form  of  which  was  described 


340  GENERAL  EMBRYOLOGY 

above,  a  region  of  more  actively  dividing  cells  can  be  distin- 
guished extending  around  the  fully  formed  blastula  just  above 
its  equator,  that  is,  between  animal  and  vegetal  poles  (Fig.  154); 
this  is  termed  the  germ  ring,  although  it  is  frequently  not  a 
complete  ring,  being  interrupted  on  the  anterior  side  of  the 
blastula.  Gastrulation  commences  by  the  true  invagination  of 
cells  just  below  the  germ  ring  on  the  posterior  side.  The  inert- 
ness of  the  large  yolk-filled  cells  of  the  vegetal  hemisphere 
greatly  impedes  the  process  of  invagination  and  it  is  never 
carried  very  far.  Involution  occurs  extensively  in  the  dorsal 


FIG.  154. — Section  through  late  blastula  of  frog,  showing  location  of  germ  ring. 
Later  the  germ  ring  is  thickened  and  contracted,  forcing  the  yolk  cells  upward 
into  the  segmentation  cavity.  After  O.  Schultze.  a,  animal  pole;  gr,  germ  ring; 
p,  pigment;  s,  blastocoel;  v,  vegetal  pole. 

region  and  is  accompanied  by  very  pronounced  epiboly  (Fig. 
155);  this  latter  process  is  very  marked  in  the  lateral  and 
ventral  regions  where  little  or  no  invagination  occurs.  All  of 
these  processes  are  relatively  less  extensive  than  in  the  coelo- 
gastrula  of  Amphioxus  however,  probably  on  account  of  the 
larger  amount  of  yolk  contained  in  all  of  the  cells.  Their  place 
in  development  is  taken,  in  a  way,  by  a  new  process,  namely, 
delamination.  This  is  a  process  of  splitting,  whereby  an 
extended  mass  or  sheet  of  cells  becomes  cleaved  apart  by  the 
rearrangement  of  the  cells  into  two  more  or  less  distinct  layers, 
separated  by  a  definite  space.  This  process  of  delamination 


E 


FIG.  155. — Series  of  diagrammatic  drawings  of  sections  showing  the  process 
of  gastrulation  in  the  Urodele,  Triton.  After  Greil  and  Ruffini.  F.  Transverse 
section,  others  sagittal.  A.  Blastula.  B.  Commencement  of  gastrulation 
(invagination).  C.  Continued  invagination  accompanied  by  epiboly  and  invo- 
lution. Formation  of  archenteron.  D.  Continuation  of  all  three  processes 
of  gastrulation.  Blastocoel  nearly  obliterated.  E.  Archenteron  completely 
formed.  Rudiments  of  notochord  and  neural  plate  differentiated.  F.  Trans- 
verse section  through  embryo  shown  in  E,  through  the  plane  marked  ff.  a, 
archenteron  (gut  cavity  in  F)  lined  with  endoderm;  6,  blastopore;  ch,  rudiment 
of  notochord;  ec,  ectoderm;  en,  endoderm;  ff,  plane  of  section  shown  in  F;  gm, 
gastral  (axial)  mesoderm;  n,  neural  plate  (in  F,  bounded  by  neural  folds);  pm, 
peristomial  mesoderm;  s,  blastoccel  or  segmentation  cavity;  x,  marks  corre- 
sponding points  on  the  surface  ectoderm,  showing  extent  of  epiboly;  y,  yolk  cells. 


342  GENERAL  EMBRYOLOGY 

begins  where  the  processes  of  invagination  and  involution  leave 
off,  and  it  is  important  to  recognize  that  the  didermic  character 
of  the  gastrula  of  the  Amphibian  results  partly  from  all  three 
of  these  processes.  The  chief  result  of  involution  is  the  forma- 
tion of  the  rudiments  of  the  notochord  and  gastral  mesoderm, 
as  in  Amphioxus.  Figure  155  shows  how  the  blastocoel  is 
finally  obliterated  by  the  invaginated  and  involuted  regions. 
The  germ  ring  finally  completes  its  growth  over  the  yolk  cells 
or  endodermal  floor  of  the  archenteron,  and  closes  together  much 
as  in  Amphioxus. 

The  formation  of  the  mesoderm  offers  some  points  of  differ- 
ence when  compared  with  Amphioxus.  The  peristomial  meso- 
derm forms  typically  in  the  margin  of  the  blastopore,  out  of 
the  undifferentiated  cell  mass  of  the  germ  ring.  Sometimes,  in 
the  region  just  within  the  blastopore  dorsally,  traces  of  entero- 
coelic  outgrowths  can  be  seen  (Fig.  156),  but  most  of  the  gastral 


ec 


FIG.  156. — Part  of  a  section  through  the  body  of  an  embryo  of  the  frog,  Rana 
fusca,  showing  traces  of  enteroccel  formation.  After  O.  Hertwig.  a,  archen- 
teron;  c,  enterocoels;  ec,  ectoderm;  en,  endoderm;  m,  mesoderm;  n,  notochord; 
p,  neural  plate;  y,  yolk  cells. 

mesoderm  is  formed  either  from  involuted  cells  derived  from 
the  germ  ring,  or  later  from  the  surface  of  the  endoderm  by  a 
process  of  delamination  or  splitting  off  of  the  superficial  cells 
lying  next  the  ectoderm;  these  come  off  first  as  a  solid  sheet, 
which  much  later  itself  splits  into  two  layers  leaving  a  coelomic 
cavity  between  them. 

Thus  while  invagination  occurs  to  a  slight  extent,  gastrula- 
tion  in  these  forms  results  largely  from  the  processes  of  involu- 


BLASTULA,  GASTRULA,  AND  GERM  LAYERS    343 

tion  and  epiboly  combined  with  delamination.  The  didermic 
condition  results,  to  a  considerable  extent,  from  the  overgrowth 
of  the  animal  hemisphere  cells  (germ  ring)  which  come  to  enclose 
the  yolk  cells  of  the  vegetal  hemisphere.  As  in  Amphioxus, 
however,  the  yolk,  although  here  so  much  more  abundant,  is 
finally  included  in  the  floor  of  the  gut  cavity,  and  the  yolk  cells 
take  a  direct  share  in  the  formation  of  the  structures  of  the 
later  embryo.  After  the  mesoderm  and  chorda  have  been 
formed  from  the  roof  of  the  archenteron,  this  is  left  as  a  thin 
layer,  only  one  cell  in  thickness,  quite  in  contrast  with  the  thick 
mass  of  cells  forming  its  floor  (Fig.  155). 

Turning  now  to  the  third  type  of  gastrula,  that  formed  from 
the  discoid  blastula,  we  find  conditions  which  vary  widely  from 
the  Amphioxus  type,  but  which  after  all  may  be  interpreted 
in  the  light  of  the  processes  just  outlined.  In  the  Ganoid  or 
Amphibian,  both  the  animal  and  vegetal  hemispheres  of  the 
egg  share  directly  in  the  processes  of  cleavage,  and  blastula  and 
gastrula  formation;  and  the  yolk,  contained  in  typical  cells,  is 
carried  directly  into  the  wall  of  the  primitive  gut.  But  in 
the  extremely  meroblastic  eggs  of  the  Elasmobranchs,  Teleosts, 
Reptiles,  and  Birds,  the  large  yolk  mass,  which  is  the  equivalent 


FIG.  157. — Sagittal  section  through  early  gastrula  of  the  catfish,  Ameiurus. 
en,  endoderm;  gr,  germ  ring;  p,  periblast;  s,  segmentation  cavity,  or  sub-ger- 
minal cavity;  y,  yolk. 

of  the  vegetal  pole  of  the  egg,  does  not  cleave  (Figs.  150,  48) 
and  takes  no  direct  share  in  the  formation  of  the  cellular  blastula 
and  gastrula.  For  comparative  purposes,  therefore,  we  have 
already  seen  that  we  must  recognize  the  germ  disc  or  "blastula" 
of  this  type  as  equivalent  only  to  the  animal  hemisphere  of  such 
a  form  as  the  frog  or  Amphioxus,  flattened  out  and  resting 
upon  the  undivided  yolk  mass.  In  such  a  condition  as  this  the 
equivalent  of  the  germ  ring  would  be  found  forming  the 


344 


GENERAL  EMBRYOLOGY 


pm 


en 


ch 


FIG.  158. — Semi-diagrammatic  drawings  of  sections  through  Elasmobranch 
embryos.  After  Greil,  after  Riickert  and  Ziegler.  A-D.  Sagittal  sections; 
E,  F,  transverse  sections.  A.  Diagram  of  section  to  show  relations  of  blasto- 
derm to  animal  pole  of  blastula  with  less  yolk.  B.  Commencement  of  gastrula- 
tion  by  invagination,  epiboly,  and  involution.  C,  D.  Continuation  of  all  three 
processes  of  gastrulation.  E.  Transverse  section  showing  relation  of  endoderm 
to  yolk,  mesoderm,  and  the  germ  ring.  F .  Transverse  section  farther  forward, 
through  stage  resembling  D.  Anterior  margin  of  blastoderm  toward  the  left  in 
A-D.  a,  archenteron;  c,  vestiges  of  enteroccels;  ch,  rudiment  of  notochord; 
ec,  ectoderm;  en,  endoderm;  gm,  gastral  (axial)  mesoderm;  gr,  germ  ring;  m, 
mesoderm;  n,  neural  groove,  bordered  laterally  by  neural  folds;  pm,  peristomial 
mesoderm;  s,  blastoccel  or  sub-germinal  cavity;  y,  yolk. 


BLASTULA,  GASTRULA,  AND  GERM  LAYERS    345 

peripJiery  of  tJie  blastodisc.  The  process  of  gastrulation  concerns 
only  the  blastodisc,  for  the  yolk  mass  takes  no  more  share  in 
this  than  in  the  later  processes  of  development;  the  gastrula 
forms  separately  from  the  yolk,  which  is  left  outside  the  embryo 
and  as  we  shall  see,  comes  into  relation  with  it  only  indirectly. 
The  blastula  or  blastodisc,  once  formed  as  a  flat  plate,  many 
cells  in  thickness,  begins  to  extend  over  the  surface  of  the  yolk 
mass.  Its  central  part  becomes  quite  thin  in  consequence,  but 


pm 


FIG.  159. — Diagrammatic  drawings  of  sagittal  sections  through  embryos  of 
Sauropsids.  After  Greil,  after  Will  and  Schauinsland.  A,  B.  Two  stages  of 
Reptile.  C.  Bird,  a,  archenteron;  ch,  rudiment  of  notochord;  en,  gut  endo- 
derm;  pm,  peristomial  mesoderm;  s,  blastocoel  or  sub-germinal  cavity;  y,  yolk. 

the  margin  of  the  disc,  or  germ  ring,  remains  thickened,  and  as 
it  advances  over  the  yolk  its  margin  becomes  slightly  involuted, 
forming  a  narrow  shelf  of  cells  on  its  inner  surface,  toward  the 
yolk  (Fig.  157).  This  inner  layer  is  the  rudiment  of  the  primary 
inner  layer.  Gastrulation  is  thus  primarily  accomplished  by 
involution.  The  margin  of  the  germ  disc  is  clearly  the  germ 
ring  or  rim  of  the  blast  opore,  although  its  form  and  relation  to 
the  yolk  are  quite  unlike  what  we  have  seen  heretofore. 

The  Elasmobranchs  and  Reptiles  afford  important  transi- 
tional conditions  here,  in  that  a  definite  process  of  invagination 
is  indicated  (Figs.  158,  159).  Invagination  is  here  limited  to 


346  GENERAL  EMBRYOLOGY 

the  posterior  margin  of  the  blastoderm,  where  the  germ  ring 
becomes  elevated  above  the  yolk  as  the  endoderm  is  folded 
under  the  ectoderm.  In  the  Teleosts  little  or  no  indication  of 
invagination  can  be  found.  In  these  forms  the  germ  ring 
extends  rapidly  over  the  yolk,  reaches  and  passes  an  equator 
of  the  egg,  and  then  as  it  continues  to  advance,  gradually  nar- 
rows, and  finally  closes  completely,  having  passed  over  the 
entire  yolk  mass  (Fig.  160).  This  overgrowth  of  the  blasto- 
derm occurs  more  rapidly  in  the  anterior  and  lateral  directions 
than  in  the  posterior  direction,  so  that  the  blastopore  finally 
closes  in  nearly  the  same  relative  position  as  in  the  frog  and  in 
Amphioxus,  i.e.,  postero-ventrally.  As  the  germ  ring  extends 
around  the  yolk,  only  a  single,  and  very  thin,  layer  of  cells  is 
left  behind  it  as  a  covering  layer.  In  the  posterior  and  postero- 
lateral  regions  alone,  is  the  involution  of  an  inner  layer  well 
marked.  It  should  be  noted  that  in  the  Teleosts  the  endoderm 
is  largely  replaced  functionally  by  a  specialized  protoplasmic 
region  on  the  surface  of  the  yolk,  known  as  the  periblast,  which 
contains  free  nuclei  derived  originally  from  those  of  the  margin 
of  the  blastodisc  (Figs.  150,  C;  157). 

During  the  later  stages  of  the  overgrowth  of  the  germ  ring, 
as  it  contracts  after  passing  the  equator  of  the  egg,  its  sub- 
stance is  payed  into  its  more  slowly  advancing  posterior  region, 
where  it  forms  a  longitudinal  median  thickening  (Fig.  160). 
This  thickened  region  of  the  blastoderm  is  the  primitive  streak, 
the  earliest  rudiment  of  the  essential  parts  of  the  embryo,  which 
gradually  differentiate  out  of  its  anterior  end. 

In  such  a  gastrula  as  this  the  endoderm  forms  a  flat  median 
plate  of  cells  lying  directly  upon  the  surface  of  the  periblast 
(yolk),  and  the  archenteron  is  present  only  virtually  as  a  narrow 
space  between  the  endoderm  and  periblast  (Fig.  157).  In  such 
a  case  the  formation  of  a  true  gut  cavity  is  independent  of  the 
formation  of  the  inner  layer,  and  occurs  later  by  a  process  of 
folding. 

The  mesoderm  is  differentiated  at  a  comparatively  early  stage, 
and  the  distinction  between  peristomial  and  gastral  mesoderm 
is  very  clear.  The  peristomial  mesoderm  appears  as  a  small 


BLASTULA,  GASTRULA,  AND  GERM  LAYERS    347 

mass  of  slowly  differentiating  cells  lying  in  the  germ  ring, 
between  the  superficial  ectoderm  of  the  blastoderm  and  the 
involuted  shelf  of  endoderm  (Fig.  158,  F).  The  gastral  meso- 
derm  is  seen  budding  off  laterally  from  the  primitive  streak 
region,  also  between  ectoderm  and  endoderm  or  even  beyond  the 
region  where  the  endoderm  is  found  (Fig.  158,  F).  Posteriorly 


FIG.  160. — Diagrams  of  the  formation  of  the  Teleost  embryo  by  confluence  of 
the  germ  ring,  and  the  growth  of  the  germ  ring  around  the  yolk.  From  Kopsch. 
A.  In  half -profile.  B.  In  profile. 

of  course  the  gastral  mesoderm  of  each  side  becomes  continuous 
with  the  peristomial  mesoderm,  and  as  peripheral  portions  of  the 
germ  ring,  where  the  three  layers  are  slowly  differentiating, 
are  continually  passing  into  the  posterior  end  of  the  primitive 
streak,  it  is  clear  that  peristomial  mesoderm  is  constantly 
becoming  gastral,  merely  through  relative  change  of  position, 
not  through  any  change  in  the  mode  of  its  formation  or  in  its 
relation  to  the  other  germ  layers. 

The  Elasmobranchs  and  Reptiles  are  again  transitional  in 
that  vestiges  of  enteroccelic  grooves  may  be  seen  as  shallow 
and  narrow  longitudinal  depressions,  either  side  of  the  mid- 
line,  in  the  region  of  which  the  formation  of  mesoderm  is  most 
rapid  (Fig.  158,  F).  In  the  Teleost,  as  in  the  Bird,  no  traces  of 
enterocoels  are  to  be  seen.  As  usual  the  notochord  forms  from 
the  cell  mass  lying  between  the  rudiments  of  the  gastral  meso- 
derm, and  may  be  said  to  have  been  derived  either  from  the 
gastral  mesoderm,  or  from  the  endoderm  in  the  same  way  that 


348  GENERAL  EMBRYOLOGY 

the  mesoderm  itself  is.  After  the  separation  of  the  chorda 
and  mesoderm,  the  endoderm  proper,  or  enteroderm,  as  it  is 
called,  is  left  as  a  thin  narrow  strip  of  cells  spread  flat  over  the 
periblast  (yolk)  surface,  continuous  posteriorly  with  the  diverg- 
ing limbs  of  the  germ  ring. 

In  the  Sauropsids,  where  the  accumulation  of  the  yolk  is  most 
pronounced,  the  blastoderm  does  not  grow  entirely  around  the 
yolk  until  long  after  the  gastrula  is  formed  and  the  embryo 
established.  Correlatively  we  find  no  typical  germ  ring  forma- 
tion in  the  periphery  of  the  blastoderm,  save  in  that  posterior 
region  which  is  to  be  concerned  in  embryo  formation.  Remem- 
bering that  in  the  Reptile  both  true  invagination  and  enter ocoel 
formation  occur,  while  these  processes  are  not  apparent  in  the 
birds,  we  may  describe  (following  Patterson's  account)  the 
processes  of  gastrulation  and  embryo  formation  in  the  pigeon, 
as  illustrating  these  events  in  the  development  of  the  extremely 
meroblastic  ovum. 

The  blastoderm  first  becomes  quite  thin,  particularly  toward 
its  posterior  side,  where,  at  the  same  time,  the  margin  thickens 
forming  a  segment  of  a  true  germ  ring  (Fig.  161).  The  exten- 
sion of  this  posterior  part  of  the  germ  ring,  however,  involves 
the  usual  processes  of  cell  multiplication  accompanied  by 
involution  and  epiboly;  there  is  no  true  invagination  here  (Fig. 
161).  The  formation  of  an  inner  layer  is  thus  limited  to  the 
posterior  region  of  the  blastoderm.  Soon,  as  this  whole  region 
extends  posteriorly,  this  segment  of  a  germ  ring  begins  to 
contract  toward  the  mid-line,  and  the  result  is  the  formation 
of  a  median  thickening  in  the  posterior  half  or  third  of  the  germ 
disc.  This  thickening  is  the  primitive  streak  (Fig.  161),  and  as 
usual  it  is  the  seat  of  the  formation  of  the  chief  embryonic  rudi- 
ments. As  in  the  Teleost,  the  primitive  streak,  formed  by  the 
gradual  fusion  of  the  lateral  halves  of  the  germ  ring,  is  obviously 
the  equivalent  of  the  blastoporal  margin  of  the  frog  or  of 
Amphioxus.  On  its  surface  is  a  shallow  longitudinal  groove 
marking  the  separation  of  the  two  halves;  this  is  the  primitive 
groove  (Fig.  161),  which  may  be  regarded  as  representing  the 
blastopore  proper.  The  archenteron,  in  such  a  gastrula  as  this 


BL-ASTULA,  GASTRULA,  AND  GERM  LAYERS    349 


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350  GENERAL  EMBRYOLOGY 

of  the  chick,  may  also  be  said  to  exist  only  virtually,  for  it  is 
represented  only  by  a  shallow  space  left  between  the  endoderm 
and  the  yolk. 

The  mesoderm  and  chorda  are  here  more  closely  related  with 
the  outer  than  with  the  inner  layer.  In  the  germ  ring  there  is 
little  indication  of  separation  of  the  germ  layers,  other  than  the 
distinction  of  the  endoderm,  and  when  the  primitive  streak  is 
formed,  it  appears  rather  as  a  thickening  of  the  ectoderm.  The 
mesoderm  begins  to  be  differentiated  along  the  sides  of  the 
primitive  streak,  and  back  as  far  as  the  region  where  this  is 
being  formed  by  the  fusion  of  the  limbs  of  the  germ  ring.  Hence 
the  mesoderm  is  more  largely  gastral,  that  is  to  say,  it  does  not 
become  distinct,  as  a  separate  rudiment,  until  the  establishment 
of  the  primitive  streak  has  occurred.  In  the  germ  ring  there 
is  of  course  a  region  where  ectoderm  passes  into  endoderm,  and 
where  the  cells  may  be  said  to  belong  to  either  layer  or  neither 
layer.  This  is  the  region  \vhere  the  primary  "mesoderm" 
forms  and  apparently  special  conditions  may  determine  with 
which  of  the  primary  layers  it  may  seem  to  have  the  more  inti- 
mate relation.  Little  is  gained  by  attempting  to  define  germ 
layers  in  the  germ  ring. 

In  the  pigeon  or  chick  the  rudiment  of  the  notochord  appears 
in  the  deeper  part  of  the  primitive  streak  after  its  lateral  parts 
are  cut  off  as  mesoderm.  The  endoderm  has  therefore  the  value 
of  an  enteroderm  from  the  beginning,  and  has  the  form  of  a  very 
thin  flat  sheet  of  cells  widely  spreading  over  the  yolk  surface. 
Ultimately  the  yolk  mass  becomes  entirely  enclosed  in  a  layer 
of  endoderm  as  well  as  by  the  other  germ  layers,  but  this  does 
not  occur  until  a  comparatively  late  stage  in  the  development 
of  the  embryo. 

In  Amphioxus  and  the  frog  w^e  have  seen  that  the  embryo  is 
formed  from  the  entire  ovum,  that  is,  the  yolk-containing  cells 
become  actually  included  within  the  wall  of  the  gut.  In  the 
Teleosts  the  yolk  mass  is  so  large,  and  so  completely  separated 
from  the  embryogenic  tissues,  that  the  embryo  may  be  said  to 
develop  upon  the  surface  of  the  yolk,  which,  enclosed  within  a 
structure  called  the  yolk  sac,  is  only  indirectly  related  to  the 


BLASTULA,  GASTRULA,  AND  GERM  LAYERS    351 

embryo  proper.  In  forms  like  the  Elasmobranchs  and 
Sauropsids,  the  accumulation  of  yolk  is  still  greater  and  the 
embryo  forms  quite  apart  from  the  yolk,  with  which  it  later 
acquires  a  secondary  relation.  In  the  Sauropsids,  after  the 
rudiments  of  the  embryo  are  well  established,  a  process  of  fold- 
ing begins  and  a  series  of  infoldings  of  the  cellular  blastoderm, 
anterior,  posterior,  and  lateral,  pinch  off  the  embryo  from  the 
yolk  mass  or  yolk  sac,  with  which  it  then  remains  only  indirectly 
connected  by  a  narrow  tube  known  as  the  yolk  stalk  which 
includes  a  portion  of  the  gut  wall  and  a  very  abundant  blood 
supply. 

In  the  Sauropsids  and  Mammals  other  folds  of  the  blastoderm  soon 
appear,  beyond  the  limits  of  the  embryo  proper,  which  result  in  the 
formation  of  a  very  special  and  highly  characteristic  structure  known 
as  the  cunnion.  And  from  the  wall  of  the  hind-gut  grows  out  another 
special  and  extra-embryonic  structure,  the  allantois.  The  formation 
and  function  of  these  extra-embryonic  structures,  together  called  the 
embryonic  appendages,  cannot  be  described  here.  They  are  of  the 
greatest  importance  in  development  and  their  presence  has  led  to  the 
application  of  the  term  Amniota  to  all  the  forms  possessing  them 
(Birds,  Reptiles,  Mammals)  while  the  other  Craniates,  without  these 
embryonic  appendages  (Cyclostomes,  Fish,  Amphibia)  are  then  known 
as  the  An  am  ma. 

On  account  of  the  difficulties  of  comparison  it  seems  wise  to 
omit  reference  here  to  the  Mammalian  gastrula  and  germ  layer 
formation.  For  in  spite  of  the  nearly  alecithal  condition  of  the 
Mammalian  ovum,  its  development  shows  marked  yolk  influ- 
ence, and  the  whole  course  of  early  development  is  complicated, 
not  only  through  the  one  time  presence  and  the  subsequent  loss 
of  yolk,  but  through  the  very  special  relations  of  the  early 
embryo,  and  particularly  the  embryonic  appendages,  with  the 
walls  of  the  maternal  cavity  in  which  development  proceeds. 

CONCRESCENCE 

We  should  consider  here,  in  a  particular  way,  a  developmental 
process  which,  besides  being  of  great  general  importance  in 
Chordate  development,  is  of  considerable  historical  interest  as 


352 


GENERAL  EMBRYOLOGY 


well.  In  the  foregoing  pages  we  have  seen  that  where  a  germ 
disc  is  formed,  its  margin,  known  as  the  germ  ring,  and  recog- 
nized as  the  homolog  of  the  lip  or  margin  of  the  blastopore,  is  of 
great  importance  in  the  formation  of  the  primary  rudiments  of 
the  embryo. 

The  His- Whitman  theory  of  concrescence  emphasizes  the 
general  importance  and  significance  of  this  relation.  First 
stated  fully  by  His,  in  1876,  the  essential  idea  of  this  theory  was 
that  each  side  of  the  germ  ring,  not  only  forms,  but  really  is, 


FIG.  162. — Diagrams  illustrating  four  stages  in  the  formation  of  the  Teleost 
embryo  and  the  growth  of  the  germ  ring  around  the  yolk  mass.  After  O.  Hert- 
wig.  e,  embryo;  gr,  posterior  margin  of  the  germ  ring;  y,  yolk  mass;  1,  2,  3,  4, 
successive  positions  occupied  by  the  germ  ring  as  it  advances  over  the  yolk. 

the  rudiment  of  the  corresponding  half  of  the  embryo,  which  is 
thus  actually  formed  by  the  approach  and  gradual,  continued 
fusion  posteriorly  of  the  germ  ring.  In  each  half  of  the  ring 
the  essential  rudiments  of  the  embryo  were  thought  to  be  al- 
ready formed,  partly  at  least,  and  the  process  of  embryo  forma- 
tion consisted  merely  or  chiefly  in  the  junction  or  addition  of 
these  two  originally  separate  halves.  The  anterior  end  of  the 
embryo  would  thus  be  formed  first,  and  embryo  formation 


BLASTULA,  GASTRULA,  AND  GERM  LAYERS    353 

would  be  complete  when  the  germ  ring  became  fully  contracted 
or  closed. 

With  some  modifications  of  a  really  fundamental  kind,  this 


D 


FIG.  163. — Diagrams  of  the  formation  of  an  embryo  by  confluence  ("con- 
crescence"). A.  Germ  ring  before  formation  of  the  embryo  is  indicated.  Tbe 
letters  a-e,  represent  symmetrical  portions  of  the  germ  ring.  B.  Beginning  of 
confluence.  C.  Embryo  forming.  A  A,  BB,  represent  regions  of  the  embryo, 
formed  out  of  the  materials  of  the  germ  ring  at  aa,  bb.  D,  E.  Later  stages  in  the 
formation  of  the  embryo.  The  germ  ring  regions,  cc,  and  dd,  have  been  differ- 
entiated into  the  embryonic  regions,  CC,  DD. 

conception  is  widely  adopted  to-day.  Both  observation  and 
experiment  have  shown,  however,  that  definite  halves  of  an 
embryo  cannot  be  said  to  exist  preformed  in  the  lateral  portions 
of  the  germ  ring.  These  regions  do  contribute  to  the  formation 


354 


GENERAL  EMBRYOLOGY 


of  the  median  thickening,  known  as  the  primitive  streak  or 
embryonic  rudiment,  by  a  process  of  gradual  fusion  posteriorly 
(Figs.  162,  163).  But  in  this  process  of  coming  together,  which 
may  better  be  termed  confluence  (Sumner)  than  concrescence, 
the  materials  from  the  two  sides  of  the  germ  ring  are  fused  into 
a  mass  which  is  largely  ^differentiated,  and  out  of  this  the 
rudiments  of  the  embryo  appear,  by  a  process  of  differentiation 
which  occurs  largely  after  confluence.  One  side  of  the  germ 
ring  contains  not  a  half  of  the  embryo,  but  the  substance  out  of 
which,  later,  a  half  of  an  embryo  forms.  This  process  of 
differentiation  is  progressive  and  commences  of  course  in  that 


en. 


FIG.  164. — Sagittal  section  through  the  hinder  end  of  a  fish  embryo  (Serranus), 
showing  the  undifferentiated  primitive  streak,  anterior  to  which  the  structures 
of  the  embryo  are  being  differentiated.  From  H.  V.  Wilson,  a.p.  (v.l.),  anterior 
margin  of  blastoderm  or  ventral  lip  of  blastopore,  after  having  grown  entirely 
around  the  yolk  mass,  bl.,  blastopore;  ec,  ectoderm;  en.,  endoderm;  g.r.,  germ 
ring;  k.v.,  Kupffer's  vesicle;  nc.,  notochord;  nr.  ch.,  nerve  cord;  p.,  periblast; 
pp.  (d.l.),  posterior  margin  of  blastoderm  (dorsal  lip  of  blastopore);  pr.  str., 
primitive  streak. 

part  of  the  primitive  streak  formed  first,  i.e.,  its  morphological 
anterior  end  (Fig.  163).  Then  as  the  primitive  streak  lengthens 
posteriorly,  the  extent  of  the  differentiated  region  at  its  anterior 
end  similarly  increases  posteriorly,  roughly  keeping  pace  with 
the  process  of  elongation  (Fig.  164).  The  primitive  streak 
thus  may  be  regarded  as  a  region  which  moves  backward, 
receiving  posteriorly  the  diverging  limbs  of  the  germ  ring,  and 
leaving  anteriorly  the  differentiated  rudiments  of  the  embryo. 
Soon  after  the  germ  ring  is  completely  closed  or  contracted, 
the  primitive  streak  becomes  wholly  differentiated,  and  the 


BLASTULA,  GASTRULA,  AND  GERM  LAYERS    355 


rudiments  of  the  embryo  may  be  said  to  be  fully  marked  out. 
The  process  of  concrescence  is  seen  most  clearly  and  typically 
in  those  forms  with  large  amounts  of  yolk  and  with  well-marked 
germ  ring,  especially  in  the  Teleosts  and  Elasmobranchs  (Figs. 
160,  165).  In  the  Amphibia,  where  the  amount  of  yolk  is  less, 
and  the  Sauropsida,  where  the  germ  ring  is  less  marked,  the  proc- 
ess of  concrescence,  though  somewhat  modified  and  slightly 
obscured,  still  takes  an  important  part  in  embryo  formation. 

THE  GERM  LAYERS 
While  this  is  not  the  place  to  give  an  historical  or  critical 


FIG.  165. — Blastoderms  of  the  Elasmobranch,  Torpedo,  showing  formation  of 
the  embryo.  After  Ziegler.  X  27.  A.  "Stage  B."  The  postero-median 
thickening  is  the  "embryonic  shield,"  the  first  indication  of  the  real  embryo. 
B.  " Stage  C."  Early  embryo;  nerve  cord  rising  above  the  surface  of  the  blasto- 
derm. In  both  figures  the  embryonic  portion  of  the  blastoderm  is  directly 
continuous  postero-laterally,  with  the  germ  ring,  which  appears  as  the  thickened 
margin  of  the  blastoderm. 

account  of  the  germ  layer  theory,  it  is  important  that  the  stu- 
dent should  have  in  mind,  before  taking  up  the  study  of  the 
development  of  particular  organisms,  certain  fundamental 
conceptions  of  the  germ  layers,  and  their  relation  to  develop- 
ment, particularly  among  the  Chordata. 

When  the  science  of  Embryology  was  itself  in  a  very  early 
stage  of  its  development,  the  earliest  differentiations  recognized 
by  the  students  of  animal  development,  were  the  sheets  or 


356  GENERAL  EMBRYOLOGY 

layers  of  tissue,  such  as  those  of  the  chick,  which  seemed  to  give 
rise  to  the  chief  organs  of  the  embryo.  These  layers  of  sub- 
stance were  described  and  their  significance  recognized,  by 
such  pioneers  in  embryology  as  C.  F.  Wolff  (1768),  Pander 
(1817),  and  Von  Baer  (1828).  The  whole  history  of  the  forma- 
tion of  the  systems  and  organs  of  the  embryo  could  be  read  back 
to  these  layers,  but  beyond  these,  few  constant  structural 
features,  susceptible  of  homolgy  in  different  forms,  could  be 
made  out.  The  idea  became  very  firmly  fixed  therefore,  that 
these  layers  were  really  the  primary  differentiations  of  the 
embryo,  and  quite  naturally  their  importance  was  strongly 
emphasized. 

Subsequent  to  the  statement  and  establishment  of  the  cell 
theory,  the  genesis  of  the  germ  layers  was  traced,  the  blastula 
and  gastrula  fully  described  in  a  great  variety  of  forms,  and  it 
was  found  that  in  spite  of  the  greatest  diversity  in  the  earlier 
processes  of  development,  the  general  character  and  structure 
of  the  germ  layers  remained  remarkably  uniform.  And  not 
only  were  the  relations  of  the  germ  layers  to  one  another  quite 
constant,  but  their  relation  to  the  tissues  and  organs  of  the 
later  embryo  were  subject  to  but  little  variation.  In  all  forms 
inner  and  outer  layers  (endoderm  and  ectoderm)  were  present, 
and  in  all  forms  above  the  Coelenterates  a  definite  intermediate 
layer  (mesoderm)  was  to  be  found.  Moreover,  in  all  of  these 
forms  the  outer  layer  gave  rise  to  the  whole  nervous  system, 
central  and  peripheral,  the  essential  parts  of  the  sense  organs, 
the  epidermis  and  its  appendages;  from  the  inner  layer  came  the 
lining  of  the  digestive  tract  and  its  glandular  appendages; 
while  the  intermediate  layer  gave  rise  to  the  sustentative, 
vascular,  and  muscular  tissues  throughout  the  body.  All  of 
this  was  finally  developed,  notably  by  the  Brothers  Hertwig 
(1879-1883),  into  a  carefully  and  elaborately  worked  out  Germ 
Layer  Theory,  the  essential  points  of  which  were  that  the  three 
germ  layers  are  entirely  homologous  throughout  the  Metazoa, 
excepting  only  the  Porifera  (the  Coelenterates  of  course  lacking 
a  middle  layer),  and  that  these  layers  truly  represent  the  pri- 
mary and  fundamental  homologies  in  the  structure  of  the 


BLASTULA,  GASTRULA,  AND  GERM  LAYERS     357 

Metazoan  phyla.  Exceptions  and  contradictions  were  indeed 
occasionally  noted,  but  their  importance  was  minimized  and 
they  were  treated  frankly  as  exceptions,  and  put  down  to  the 
account  of  "  ccenogenetic  "  modifications  of  "  palingenetic " 
characteristics  (see  Chapter  I). 

It  is  difficult  to  overestimate  the  influence  of  this  theory  upon 
the  history  of  Embryology,  and  upon  fundamental  embryo- 
logical  ideas.  Perhaps  no  conception,  other  than  the  general 
theory  of  evolution,  has  had  greater  influence  hi  the  field  of 
descriptive  embryology. 

More  recently,  however,  the  limitations  in  the  general 
applicability  of  this  theory  have  been  more  fully  recognized 
and  the  exceptions  to  the  validity  of  its  essential  ideas  empha- 
sized. At  present  we  must  recognize  the  germ  layers  as 
representing  a  stage  in  development,  just  as  do  the  blast ula  or 
gastrula,  and  of  no  greater  or  lesser  importance  than  these. 
The  germ  layers  are  descriptive  terms  of  the  greatest  impor- 
tance, as  such  they  are  indispensable.  They  are  not,  however, 
starting  points  in  any  real  sense;  and  to  regard  them  as  such  is 
to  look  forward  merely,  not  backward.  Looking  both  forward 
and  backward  we  see  that  the  establishment  of  the  germ  layers 
is  only  one  step  in  the  continuous  process  of  development. 
They  represent  no  more  essential  homologies  than  many  other 
features  held  in  common  by  many  developing  organisms. 

While  we  cannot  consider  in  extenso  the  facts  which  have  led 
to  this  change  of  opinion  regarding  the  importance  of  the  germ 
layers,  we  are  bound  to  state  the  nature  of  certain  classes  of 
these  facts.  In  the  first  place  are  to  be  noted  the  difficulties 
of  homologizing  the  layers  of  certain  groups  with  their  typical 
condition.  For  example,  in  the  Porifera  that  layer  which  seems 
entitled  to  be  termed  the  ectoderm,  really  gives  rise  to  struc- 
tures ordinarily  derived  from  endoderm,  while  the  "endoderm" 
itself  forms  the  covering  tissues.  In  the  Mammals  the  "ecto- 
derm" may  contribute  little  or  nothing  to  the  formation  of  the 
real  embryo  and  the  inner,  outer,  and  middle  layers  cannot 
be  exactly  homologized  with  those  of  other  Chordates,  save  by 
the  grace  of  terminology.  In  the  earlier  part  of  this  chapter 


358  GENERAL  EMBRYOLOGY 

the  varied  relations  of  the  mesoderm  to  the  other  layers  were 
mentioned;  in  some  cases  the  middle  and  inner  layers  arise 
from  a  common  rudiment,  in  others  the  middle  and  outer 
layers.  Among  the  Invertebrates  there  are  many  instances  of 
development  where  even  the  two  primary  layers  are  to  be  made 
out  only  with  considerable  difficulty,  as  for  example,  in  the 
Trematodes,  Cestodes,  certain  of  the  Bryozoa,  etc. 

In  the  second  place  the  morphogenetic  value  of  the  individual 
layers  is  subject  to  a  considerable  variation.  Thus  in  the 
Chordata,  leaving  aside  the  Mammals,  the  mesenchymal  con- 
nective-tissue cells  may  be  occasionally  of  "ectodermal"  or 
"endodermal/'  as  well  as  of  "mesodermal"  origin.  The  endo- 
thelium  of  the  heart  may  be  "endodermal"  or  "mesodermal." 
The  notochord  may  with  equal  correctness  be  described  as 
endodermal,  mesodermal,  or  even  ectodermal,  in  various  forms. 

Single  organs  like  the  nephridia  may  be  composites,  ecto- 
dermal and  mesodermal,  or,  in  some  cases  ectodermal,  in  others 
mesodermal. 

In  the  process  of  regeneration  certain  contradictions  to  the 
germ  layer  theory  become  apparent.  Organs  and  tissues  nor- 
mally derived  during  embryonic  development  from  a  certain 
layer  may,  during  regeneration,  be  produced  from  another 
layer.  In  certain  Oligochsetes  new  mesoderm  is  of  ectodermal 
origin,  and  the  regenerated  pharynx  may  be  lined  with  endo- 
dermal, rather  than  ectodermal  cells. 

Especially  in  the  process  of  budding,  as  it  occurs  in  a  great 
many  groups,  do  we  find  abundant  exceptions  to  this  theory. 
In  some  of  the  Polyzoa  the  gut  may  be  of  ectodermal  origin; 
the  nervous  system  and  pharynx  are  mesodermal  in  some  of 
the  flatworms.  Analogous  conditions  are  very  common  among 
the  Tunicates;  here  the  pharynx  may  be  endodermal  or  ecto- 
dermal; the  atrium  and  even  the  nervous  system  may  be  ecto- 
dermal, mesodermal,  or  endodermal,  in  different  forms  where 
in  egg  development  the  relations  of  these  structures  to  the 
germ  layers  are  typical. 

Finally,  the  most  important  qualifications  and  limitations  of 
the  germ  layer  theory  grow  out  of  the  observed  facts  of  normal 


BLASTULA,  GASTRULA,  AND  GERM  LAYERS    359 

development  prior  to  the  formation  of  the  germ  layers.  These 
structures  are  by  no  means  the  earliest  constant  embryonic 
differentiations,  and  as  we  have  seen  in  the  chapter  on  cleavage, 
it  is  just  as  easy  to  draw  homologies  between  cell  groups  in  the 
blastula  stage,  or  in  an  earlier  cleavage  stage,  as  it  is  between 
the  later  appearing  germ  layers.  It  is  not  too  much  to  say 
that  in  some  cases  homologies  may  be  drawn  between  various 
formed  substances  in  the  undivided  egg.  Animal  and  vegetal 
poles  of  the  ovum,  cleavage  patterns,  cell-groups,  micromeres, 
macromeres,  upper  and  lower  poles  of  the  blastula,  are  all 
constant  and  comparable  features  of  development  no  less  than 
inner,  outer  and  middle  germ  layers.  We  may  recall  that /the 
cell  known  as  4d  may  be  identified  and  its  history  and  fate 
compared,  in  the  cleavage  of  many  groups,  even  in  different 
phyla.  This  cell  whose  form,  position,  and  derivation  are  so 
constant,  may  or  may  not  form  "mesoderm";  even  when  it 
does  form  mesoderm  this  may  go  to  form  very  different  parts  of 
the  embryonic  and  adult  structure.  Often  the  "  mesoderm " 
may  be  a  cell,  just  as  truly  as  a  layer. 

Summarizing  we  may  say  that  while  the  arrangement  of  the 
cells  of  the  embryo  in  the  form  of  definite  layers  is  almost 
universal,  at  the  same  time,  in  the  comparison  of  different 
groups  or  of  different  modes  of  development,  these  layers 
exhibit  great  inconstancy  in  their  relations  to  one  another, 
and  to  the  structures  forming  them  and  formed  from  them. 
The  germ  layers  are  valuable,  indeed  indispensable  descriptive 
units,  but  they  do  not  represent  primary  differentiations,  and 
their  homologies  are  no  more,  though  probably  no  less,  funda- 
mental throughout  groups  larger  than  phyla,  than  are  many 
other  structures  of  the  developing  organism. 

MORPHOGENETIC  PROCESSES 

It  remains  now  to  describe  some  of  the  more  general  processes  by 
which  the  rudiments  of  the  organs  and  tissues  of  the  embryo  may  be 
formed  out  of  the  layers  or  cell  masses  of  the  gastrula  and  post-gastrula 
stages.  We  shall  not  attempt  to  describe  here  the  actual  formation  of 
any  specific  embryonic  structure,  but  rather  shall  give  a  brief  classifica- 


360  GENERAL  EMBRYOLOGY 

tion  of  the  more  common  and  important  processes,  a  few  of  which  have 
already  been  mentioned  earlier  in  this  chapter. 

While  the  morphogenetic  processes  within  the  embryo  show  the 
greatest  diversity  and  vary  almost  infinitely  in  specific  details,  yet  it  is 
possible  to  include  them  all  under  a  few  heads,  when  these  matters  of 
detail  are  omitted.  Again  it  should  be  recalled  that  we  are  limiting 
our  description  to  the  Chordata. 

The  primary  condition  of  morphogenesis  is  cell  multiplication.  After 
each  division  the  daughter  cells  increase  to  practically  the  size  of  the 
parent  cell;  and  numerical  increase  in  cells,  together  with  their  growth, 
i.e.,  cell  proliferation,  play  either  a  primary  or  a  secondary  part  in  every 
morphogenetic  process.  When  the  process  of  cell  division  is  quite 
general  throughout  the  extent  of  the  germ  disc  or  layer,  the  result  is  an 
increase  in  the  thickness  or  in  the  extent  of  the  sheet,  respectively,  when 
the  plane  of  the  cell  divisions  is  in  general  parallel  with,  or  perpendicular 
to,  the  plane  of  the  whole  layer.  If  there  should  be  little  or  no  regularity 
in  the  positions  of  the  division  planes,  the  membrane  would  increase  in 
al]  directions  (Fig.  166). 

Ordinarily,  in  embryogeny,  cell  multiplication  and  growth  are  more 
intense  in  restricted  areas  of  the  blastoderm  or  germ  layer.  It  is 
convenient  then  to  distinguish  between  (a)  those  processes  in  which  the 
multiplying  cells  tend  to  remain  associated  in  the  same  general  region, 
and  (fr)  other  processes  where  they  become  more  or  less  separated,  either 
from  one  another  or  from  their  seat  of  origin.  Under  the  former  head 
we  must  again  distinguish  between  the  results  of  increase  in  thickness 
and  in  extent.  A  localized  increase  in  thickness  is  frequently  termed  a 
bud;  buds  may  project  either  above  (limb  bud)  or  below  (Teleostean  lens) 
the  free  surface  where  they  are  formed.  If  the  thickening  region  is 
elongated  the  result  may  be  the  formation  of  a  strand  or  plate  of  cells, 
again  either  a  ridge-like  structure  above  the  surface  of  the  layer  (genital 
ridge),  or  a  keel-like  thickening  below  the  surface  (Teleostean  nerve 
cord,  in  part). 

Increase  in  extent  of  a  localized  area  frequently  involves  the  obstruc- 
tive action  of  the  region  bounding  the  area.  When  this  form  of  growth 
occurs  generally,  so  that  the  tendency  to  extension  occurs  in  every 
direction  from  the  middle  of  the  area  concerned,  the  result  is  frequently 
an  arching,  either  outward  or  inward.  This  may  take  the  form  of  a 
simple  arching  as  in  the  Teleostean  blastula  (Fig.  150,  C),  or  the  same 
process  may  be  carried  farther  and  followed  by  a  constriction  near  the 
base.  Such  processes  are  very  common  indeed  and  are  termed  in- 
vagination  and  evagination,  according  as  the  growth  is  below  or  above 
the  free  surface.  Simple  illustrations  of  evagination  are  afforded  by 
the  formation  of  intestinal  villi,  the  rudiments  of  lung  or  thymus,  and 


BLASTULA,  GASTRULA,  AND  GERM  LAYERS    361 

the  like;  typical  invaginations  are  seen  in  the  formation  of  the  optic 
cup  out  of  the  optic  lobe,  the  auditory  sac,  etc.  When  this  form  of 
growth  in  extent  is  limited  to  certain  directions  instead  of  occurring 
radially,  the  result  is  often  the  formation  of  a  fold  which  bears  some- 
what the  same  relation  to  the  dilation  that  the  strand  does  to  the  bud. 
The  fold  also  may  be  above  the  surface  of  the  membrane,  forming  a 
sort  of  arch  or  hollow  ridge,  usually  bounded  by  lateral  depressions 
(frog's  pronephric  duct),  or  below  the  surface  forming  a  groove  or  furrow 
bordered  by  lateral  elevations  (medullary  groove)  (Fig.  167).  In  some 
instances  a  solid  strand  may  be  formed  in  this  way  instead  of  by  the 
simpler  process  of  direct  increase  in  thickness  (Teleostean  nerve  cord, 


FIG.  166. — Diagrams  of  four  stages  in  the  formation  of  an  epithelial  thickening, 
several  layers  of  cells  deep.     From  Korschelt  and  Heider. 

in  part).  When  the  processes  leading  to  the  formation  of  a  groove  are 
continued,  the  groove  may  be  converted  into  a  closed  canal  by  the 
approach,  apposition,  and  fusion  of  the  borders  (neural  tube)  (Fig.  167). 

In  those  conditions  where  the  proliferating  cells  become,  to  some 
extent,  separated  either  from  one  another  or  from  the  proliferating 
region  itself,  we  may  note  first,  instances  of  actual  cell  migration. 
This  may  be  either  emigration  or  immigration,  according  as  to  whether 
we  fix  attention  upon  the  source  or  the  destination  of  the  migratory 
cells  (mesenchyme  cells) .  Secondary  processes  of  thickening  or  thinning 
may  accompany  these  processes.  In  other  cases  the  movement  of  cells 
may  be  described  as  rearrangement  rather  than  migration;  this  may  be 
illustrated  by  the  formation  of  mesodermal  somites  and  blood  islands 
(chick). 

One  of  the  common  morphogenetic  processes  is  a  combination  of 
increase  in  thickness  and  cell  rearrangement,  such  as  the  usual  forma- 


362 


GENERAL  EMBRYOLOGY 


tion  of  the  notochord,  or  the  process  of  delamination,  which  consists 
in  the  splitting  of  a  single  thickened  sheet  into  two  separate  layers, 
either  as  a  whole  or  in  localized  areas  (formation  of  mesoderm  in  the 
frog,  or  division  of  the  mesoderm  into  somatic  and  splanchnic  layers) ; 
in  some  instances  the  initial  thickening  may  not  be  very  apparent. 

Occasionally  cells  of  different  layers,  or  of  different  rudiments,  meet 
and  fuse,  forming  a  continuous  rudimentary  mass  (pituitary  body) . 


c 


FIG.    167.  FIG.    168. 

FIG.  167. — Diagrams  of  the  formation  of  the  medullary  canal  or  neural  tube,  in 
the  Vertebrates.  From  Korschelt  and  Heider. 

FIG.  168. — Diagrams  of  the  formation  of  the  mouth  and  stomodseum  in  a 
typical  form.  From  Korschelt  and  Heider.  ec,  ectoderm;  md,  pharynx; 
vd,  stomodaeum. 

Finally  we  may  mention  certain  morphogenetic  processes  of  a  wholly 
different  kind,  namely,  resorption,  and  changes  in  the  form  and  size  of 
cells.  From  this  point  of  view  merely,  the  process  of  resorption  may  be 
regarded  as  the  reverse  of  proliferation.  Definite  rudiments  may  appear 
first  as  spaces  thus  formed  by  the  gradual  dissolution  and  absorption  of 
cells  in  certain  areas.  Thus  the  oral  and  anal  openings,  gill-clefts,  etc., 
are  usually  formed  as  "perforations"  by  the  resorption  of  areas  where 
previously  separate  and  continuous  layers  became  united  by  a  process 
of  fusion  (Fig.  168).  In  other  cases  rudiments  once  established  may 


BLASTULA,  GASTRULA,  AND  GERM  LAYERS    363 

gradually  disappear,  either  wholly  (tail  of  tadpole),  or  in  part 
(pronephros)  remaining  then  as  vestigial  organs. 

Changes  in  cell  form  and  size  chiefly  fall  under  the  head  of  tissue 
differentiation,  or  histogenesis,  but  in  a  few  instances  such  processes  are 
primarily  involved  in  the  formation  of  rudiments.  The  development 
of  the  eye  affords  illustrations  of  these  processes;  the  lens  forms  as  a 
thickening  of  the  cells  on  one  side,  and  the  thinning  on  the  other,  of 
a  sac  originally  formed  by  inv agination,  and  the  optic  lobe,  at  first 
nearly  spherical,  flattens  and  invaginates,  one  layer  becoming  thickened 
as  the  chief  part  of  the  retina  (later  complicated  by  cell  proliferation) 
while  the  other  layer  forms  a  thin  pigmented  layer. 

These  processes  of  morphogenetic  value  are  tabulated  in  the  accom- 
panying summary;  the  arrangement  here  is  merely  a  convenient  one 
and  has  no  other  significance. 

It  is  extremely  important  to  recognize  that  these  morphogenetic 
processes  described  above  are  not  merely  simple  mechanical  processes. 
The  arrangement  and  behavior  of  the  cells  in  an  invaginating  or  delami- 
nating  region  are  determined  by  other  factors  than  those  of  physical 
resistances,  attractions,  etc.  These  events  are  both  in  fundamentals 
and  in  details  to  be  regarded  as  active  phenomena  of  a  living  organism. 
They  are  frequently  also  to  be  understood  from  the  historical  or  adaptive 
points  of  view. 

What  the  precise  conditions  are  which  determine  the  nature  of 
these  events,  may  usually  only  be  conjectured.  In  some  cases  they  may 
result  from  osmotic  conditions,  absorption  of  water,  etc.  But  for  the 
most  part  the  nature  of  the  specific  stimuli,  and  the  conditions  within 
the  layer  or  cell  group,  which  lead  to  the  definite  reactions  of  rudiment 
formation  are  unknown.  And  in  this  particular  field  of  development  we 
can  do  little  more  than  to  describe  what  happens  from  the  morphological 
viewpoint. 


364  GENERAL  EMBRYOLOGY 

SUMMARY  OF  THE  CHIEF  MORPHOGENETIC   PROCESSES 
I.  Cell  division  and  growth  throughout  the  layer  or  disc,  resulting  in 

(a)  Increase  in  thickness. 

(b)  Increase  in  extent. 

(c)  Increase  in  both  thickness  and  extent. 

II.  Cell  division  and  growth  localized  in  restricted  areas  of  the  layer 

or  disc. 

A.  Cells  remain  related  and  in  continuity 

(a)  Increasing  in  thickness. 

1.  Radially — formation  of  buds    <   t 

|  hollow. 

2.  Chiefly  in  one  axis — formation  of  strands  ]  j/-    i   * 

(6)  Increasing  in  extent. 

(Dilation. 
Invagination. 
Evagination. 

(Groove. 
Folds. 
Tube  (strand  secondarily). 

B.  Cells  become  separated  to  a  varying  degree. 

f  Immigration    1   with  corresponding 
(a)   Migration  <   -^    .       ,.  ;  ,.  .  ,      .  j  ,,. 

[  Emigration      J   thickening  and  thinning. 

(ft)  Rearrangement. 

(c)  Delamination  (sometimes  preceded  by  thickening). 

(d)  Fusion. 

III.  Resorption. 

(a)  Accompanying  a  process  of  perforation. 
(6)  Disappearance  (degeneration). 

IV.  Changes  in  cell  form  and  size. 

(a)  Thickening. 
(6)  Thinning. 

REFERENCES  TO  LITERATURE 

CERFONTAINE,  P.,  Recherches  sur  le  developpement  de  PAmphioxus. 

Arch.  Biol.     22.     1906. 
DAVENPORT,    C.    B.,    Studies   in    Morphogenesis.     IV.  A   preliminary 

Catalogue  of  the  Processes  concerned  in  Ontogeny.   Bull.     Mus. 

Comp.  Zool.     Harvard  Coll.     27.     1896. 
EYCLESHYMER,  A.  C.,  The  Formation  of  the  Embryo  of  Necturus,  with 

Remarks  on  the  Theory  of  Concrescence.     Anat.  Anz.     21.     1902. 
GREIL,  A.,  Ueber  die  erste  Anlage  der  Gefasse  und  des    Blutes  bei 


BLASTULA,  GASTRULA,  AND  GERM  LAYERS    365 

Holo-  und  Meroblastiern.     Verb.  Anat.  GeselL,  in  Anat.  Anz.     32. 

1908. 
HERTWIG,    O.,    Die    Lehre   von    den    Keimblattern.     Handbuch,    etc. 

I,  1,  1.     1903  (1906). 
HERTWIG,    O.,   und   R.,   Studies   on   the   Germ   Layers.     Jena.   Zeit. 

13-16  (6-9).     1879-1883. 
His,  W.,  Untersuchungen  liber  die  Entwickelung  von  Knochenfischen, 

besonders  iiber  diejenige   des   Salmens.     Zeit.   Anat.   Entw.     1. 

1876.     Untersuchungen  liber  die   Bildung  des   Knochenfischem- 

bryo.     Arch.  Anat.  Entw.     1878. 
JENKINSON,  J.  W.,  (Ref.  Ch.  VII.) 
KEIBEL,  F.,  Die  Gastrulation  und  die  Keimblattbildung  der  Wirbeltiere, 

Ergebnisse  Anat.  u.  Entw.     10.     1900  (1901). 
KOPSCH,  F.,  Untersuchungen  liber  Gastrulation  und  Embryobildung 

bei  den  Chordaten.     I.  Die  Morphologische  Bedeutung  des  Keim- 

hautrandes  und  die  Embryobildung  bei  der  Forelle.     Leipzig.  1904. 
KORSCHELT  UND  HEiDER,  Lehrbuch,  etc.     I  Abschnitt.     Experimentelle 

Entwicklungsgeschichte.     Jena.     1902.     Ill  Abschnitt.  Furchung 

und  Keimblatterbildung.     Jena.     1909-1910. 
LILLIE,  F.  R.,  The  Development  of  the  Chick.     New  York.     1908. 
PATTERSON,  J.  T.,  On  Gastrulation  and  the  Origin  of  the  Primitive 

Streak   in   the  Pigeon's  Egg. — Preliminary   Notice.     Biol.    Bull. 

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Verlauf  der  Entwicklung.     Arch.  mikr.  Anat.     66.     1900. 
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INDEX 


(The  numbers  in  black-faced  type  refer  to  pages  with  illustrations.) 


Achromatic  figure,  51 

acrosome,  of  spermatozoon,  98 

Actinophrys,  conjugation,  192;  matu- 
ration, 156 ;  plastogamy,  189 

Actinosphosrium,  chromosome  num- 
ber, 66 

Adolphi,  166 

agglutination,  of  spermatozoa    167 

alecithal  ova,  93,  226 

allantois,  351 

allosome,  310 

alveoli,  of  protoplasm,  36 

Amblystoma,  spermatophore,  106 

Ameiurus,  gastrula,  343 

Amia,  cleavage  (incomplete"),  241 

amitosis,  43 

amnipn,  351 

Amniota,  351 

Amoeba,  autogamy,  191 ;  conjugation, 
192;  cytotropy,  189;  fission,  2; 
karyosomes,  60,  61 

Amoeba  diploidea,  maturation  di- 
visions, 160 

amphiaster,  51;  mechanism  of  for- 
mation, 80 

Amphibia,  blastula,  331;  gastrula- 
tion,  339,  ff.;  341 

amphigony,  12 

amphimixis,  213 

Amphioxus,  blastula,  220,  330,  331 ; 
cleavage  (radial),  235;  fertiliza- 
tion, 175;  gastrulation,  333,  ff., 
335;  notogenesis  and  mesoderm 
formation,  337,  338;  oocyte,  90; 
spawning  time,  103 

Amphitrite,  ovary,  diagram,  110 ; 
spawning  season,  103 

Amphiuma,  spermatogenesis,  124 ; 
spermatozoon,  99 

amplexus,  105 

Anamnia,  351 

anaphase,  53 

Anasa,  chromosomes,  68; in 

relation  to  sex,  307,  ff.;  spermato- 
genesis, 308 

Andrews,  149 

animal  pole,  of  ovum,  92 

anisogamy,  192,  196,  ff.;  facultative, 
196,  197 

annulus,  of  spermatozoon,  100 


Apus,  parthenogenesis,  203 

Arbacia,  egg  structure.  275,  276 

Arcella,  plastogamy,  190 

archenteron,  334 

archoplasm,  38 

Argonaut  a,  micropyle,  96 

Aristotle,  1 

armadillo,  multiple  embryos,  315 

Artemia,  maturation,  158 

Ascaris,  cell  lineage,  253,  255; 
chromatin  elimination  during 
early  cleavage,  65;  chromosomal 

continuity,  72,  74 ; variation 

(numerical),  66,  67;  chromosome 
groups  during  cleavage,  223; 
cleavage,  254,  257;  fertilization, 
182;  fused  ova,  282;  "giant" 
polar  bodies,  148,  149;  idiochro- 
mosomes,  311;  primordial  germ 
cells,  112;  segregation  of  germ, 
133;  spermatozoon,  99;  tetrad 

formation,  in  oogenesis,  145 ; 

in  spermatogenesis,  137 

asexual  reproduction,  12 

aster,  39,  49 

Asterias,  chromosomes  from  chro- 
matin reservoir  (nucleolus),  71; 
development  of  parts  of  gastrula, 
283 

Astropecten,  maturation  in  partho- 
genesis,  159 

attraction  cone,  168; sphere,  39 

autogamy,  191 

autonomy  of  male  and  female  chro- 
mosome groups,  223 

axial  filament,  of  spermatozoon,  98 

von  Baer,  91,  355 

Balfour,  93; Law  of  Cleavage, 

230 

Baltzer,  75,  172,  303,  304,  311,  314 
basal  cells,  121 
Bataillon,  303 
bee,  relation  of  sex  to  fertilization, 

315 
Begonia,  reproduction  from  cuttings, 

165 

van  Beneden,  41,  65,  66,  80,  91,  131 
Bernstein,  210 
Beroe,  cleavage  (disymmetrical),  238 


367 


368 


INDEX 


Bigelow,  230 

Bilharzia,  in' copula,  106 

Biogenetic  Law,  25,  ff. 

biophores,  292 

Birds,  gastrula  and  gastrulation, 
345,  349 

blastoccel,  221,  331 

blastoderm,  241,  332 

blastodisc,  240,  332 

blastomeres,  homologies,  256,  257; 
isolated,  development  of,  269,  ff., 
280,  ff.;  nomenclature,  248,  ff. 

blastopore,  334 

blastula,  19,  221,  330;  types,  220, 
221 

Bodo,  conjugation,  196 

Bonnet,  22 

Boring,  311 

Born,  281 

Boveri,  70,  74,  81,  137,  300,  301,  305, 
311 

Brachystola,  chromosomal  individu- 
ality in  spermatogonium,  73; 
chromosomes,  68,  73 

Brauer,  158 

breeding  habits,  102,  ff. 

bridges,  intercellular,  32,  42 

brood  cavities,  104 

brood  formation,  3;  in  Mycetozoa, 
etc.,  190 

budding,  6 

bud-fission,  7 

Buller,  166,  167 

Butschli,  81,  209 

Bythotrephes,  spermazotoon,  108 

Calcium,  effect  upon  blastomeres, 
318-319 

calcium-free  sea  water,  effects  upon 
cleavage  stages,  271-272 

Calkins,  191,  210;  and  Cull, 

64,  157,  296 

Canthocamptus,  intranuclear  spindle, 
67 ;  oogenesis,  116 

Cavia,  metamorphosis  of  spermatid, 
126 

cell,  continuity,  42;  definition,  31; 
diagram  of,  35;  displacement,  in 

cleavage,  231 ;  division,  43,  ff.; 

causes  of,  76,  ff.;  -  -  indirect, 
44,  ff.;  -  -  mechanism,  79,  ff.; 
—  plane,  55,  56  ;  forms,  32,  33 ; 
general  account,  31,  ff.;  interaction 
hypothesis,  263-264;  lineage,  247; 
migration,  in  morphogenesis,  361; 
multiplication,  in  06-  and  sper- 
matogenesis,  113,  114;  polarity, 
41;  proliferation,  in  morphogene- 
sis, 360;  structure,  34,  ff.;  Theory, 
in  Embryology,  20;  wall,  34 


central  body,  61 

centrifugal  force,  effects  upon  egg 
structure,  275,  ff. 

centrolecithal  ova,  94,  226 

centronucleus,  61 

Centropyxis,  nuclear  division,  60 

centrosome,  38,  ff.;  "artificial,"  183, 
207;  continuity,  during  fertiliza- 
tion, 181,  183;  formation  by 
spermatozoon,  181;  in  Protozoa, 
60,  ff.;  origin,  in  Protozoa,  60-62 

centrosphere,  39,  125 

Ceratocephale,  spawning  season,  103 
.  Cercomonas,  conjugation,  193 

Cerebratulus,  cleavage,  236 ;  develop- 
ment of  isolated  blastomeres,  287 ; 
fertilization,  184 ;  maturation  of 
ovum,  143  ;  organization  of  ovum, 
development  of,  287 

Cerfontaine,  333 

Chcetopterus,  differentiation  without 
cleavage,  279,  279;  organization 
of  ground  substance  of  ovum,  278; 

ovum,  268 ;  polarity  of 

ovum,  275;  structure  of  ovum 
following  fertilization,  268 

chemicals,  effects  of,  upon  differen- 
tiation, 318,  ff. 

Chilomonas,  cell  division,  59,  64 

Chimcera,  egg  case,  109 

Chlamydomonas,  conjugation,  193, 
194;  gametes,  198 

Chlamydophrys,  nuclear  division,  61 

chondriosomes,  41 

"chondromiten,"  41 

chordaplasm,  267 

chorion,  96,  120 

chromatin,  38;  elimination,  65;  ex- 
trusion, 45;  possible  signifi- 
cance, 298;  granules,  293,  294; 
nucleoli,  38;  reservoir,  71 

chromidia,  41,  156 

chromioles,  38;  in  reduction,  153 

chromosomes,  49-51; accessory,  310; 
as  determiners  in  differentiation, 
289,  ff.;  as  factors  in  heredity,  291, 
ff.;  behavior,  as  related  to  Men- 
delian  heredity,  294,  295;  - 
during  cleavage,  223,  224 ;  - 

maturation  and  fertilization, 

diagram,  154 ;  —  —  mitosis, 
63,  ff.;  bivalent,  67,  135;  changes 
in  volume  (growth),  69;  con- 
stancy of  form  and  size,  67,  68; 
during  interkinesis,  64,  70,  72; 
evolution,  296;  genetic  continuity, 
70,  ff.;  heterotropic,  310;  "hy- 
pothesis," 293,  ff.;  individuality, 
70;  in  heredity,  322;  in  Protozoa, 
59,  60,  64;  numbers,  66;  in 


INDEX 


369 


artificial  parthenogenesis,  207;  nu- 
merical constancy,  66; varia- 
tion, 66-67;  pairing,  68-69,  68; 
plurivalent,  67;  relation  to  sex, 
306,  ff.;  specific  constancy,  65; 
specificity,  70;  structure,  291- 
292,  291 ;  univalent,  67 

cicatrix,  120 

Ciliates,  mutual  fertilization,  194, 
196 

Cirripedia,  complemental  males,  106 

Cladonema,  germ  cells,  110 

Clai'a,  blastula,  220 

cleavage,  18,  219,  ff.;  adequal,  227; 
bilateral,  235-237;  complete,  226, 
232;  determinate,  244;  deviations 
from  "laws"  of,  231,  ff.;  dexio- 
tropic,  235;  discoid,  227,  240; 
disymmetrical,  238;  equal,  226; 
forms,  226,  ff.;  holoblastic,  227; 
incomplete,  226,  239;  indetermi- 
nate, 244;  irregular,  239;  laeo- 
tropic,  235;  "laws"  of,  229,  ff.; 
meroblastic,  227;  nomenclature  of 
blastomeres,  248,  ff.,  270;  partial, 
227;  plane,  first,  location  of,  246, 

ff.; meridional,  vertical,  etc., 

228-229; relation  to  position 

of  centrosome,  230,  56 ;  — 
to  symmetry  of  ovum  and  adult, 
246;  processes  of,  244,  ff.;  radial, 
228,  232;  rate,  230;  rhythms,  232; 
rotatorial,  232;  spiral,  232,  234; 
superficial,  227,  241-243;  termina- 
tion of  period  of,  221;  under  pres- 
sure, 281-282,  284;  unequal,  227 

Clepsine,  cleavage  (radial),  235 

Clytia,  development  of  isolated 
blastomeres,  280 

coalescence,  development  after,  282— 
283 

Coccidium,  reproduction,  200;  schizo- 
gony,  4 

cod  fish,  number  of  ova,  107 

coeloblastula,  220,  330,  331 

ccelom,  336 

coenobium,  8 

coenogenetic  traits,  27 

Collozoum,  gametes,  198 

complemental  males,  106 

concrescence,  347,  351,  ff.,  352 

confluence,  347,  347,  351,  362,  353, 
354 

conjugation,  epidemics  of,  209;  re- 
lation to  reproduction,  208,  ff.; 
see  also  fertilization. 

Conklin,  206,  220,  258,  267,  269, 
271,  275,  278,  298,  305 

connecting  fibers,  53 

contraction  phase,  135 


Copepod,  chromosome  groups  dur- 
ing cleavage,  223,  225 

Copromonas,  conjugation,   192,   193 

copulation,  106 

cortical  layer,  90 

Crampton,  271 

Crepidula,  blastomeres,  256 ;  differ- 
entiation of  cilia,  298;  synthesis 
of  nuclein,  206 

Cristatella,  statoblasts,  8 

Crustacea,  spermatozoa,  99,  108 

Ctenophore,  cleavage  (disymmetri- 
cal), 238,  238 

Cumingia,  development  after  cen- 
trifuging,  277,  277 

cyclopia,  in  Fundulus  (artificial), 
318,  319 

Cyclops,  blastula  showing  primitive 
germ  cells,  112;  segregation  of 
germ,  133 

Cydosporia,  gametes,  198 

Cynthia,  development  of  single  blas- 
tomeres, etc.,  269,  270,  272;  non- 
correspondence  of  cell  boundaries 
and  formative  stuffs,  278;  or- 
ganization of  ovum,  267;  structure 
of  ovum  preceding  and  following 
fertilization,  176,  177,  178;  test- 
cell  nuclei  in  ovum,  120 

Cypris,  parthenogenesis,  203;  sper- 
matozoon, 100 

cytoplasm,  37;  localization  in,  264, 
ff.;  of  ovum,  differentiation  in, 
90,  ff. 

cytotropy,  189 

Dallingeria,  anisogamy,  198 

Daphnia,  spermatozoon,  108 

Dean,  109 

Delage,  207 

delamination,  340,  362 

Delia  Valle,  67 

Dentalium,  development  of  isolated 

blastomeres,  271,  273:  yolk  lobe 

(polar  lobe),  271 
determinate    cleavage,    relation    to 

indeterminate,  284,  ff. 
"determiners,"     in     differentiation, 

290,  297;  possible  nature  of,  298 
duteoplasm,  40 
development  and  differentiation,  as 

interaction,  320-321;  as  reaction 

(behavior),  24-25, 261-262;  phases 

of,  18-19 

dicentric  system,  79 
Didelphys,  spermatozoon,  99 
didermic  organism,  332-333 
Diemyctylus,  spermatophore,  105 
differentiation,    conditions   of,   262, 

ff.;  cytoplasm  and  nucleus  in,  305; 


370 


INDEX 


determination  of,  321;  role  of 
external  factors  in,  317,  ff.;  with- 
out cleavage,  in  Chcetopterus,  279, 
279 

digametic,  females,  314;  males,  314 
Dileptus,  nuclear  division,  58 
diploid  number  of  chromosomes,  133 
Diplozoon,  107 
direct  cell  division,  43 
discoblastula,  220,  331,  332 
Discoglossus,  spermatozoon,   100 
dispermy,  effect  upon  differentiation, 
300;  evidence  from,  upon  chromo- 
some hypothesis,  301 
distributed  nucleus,  58—59 
Dixippus,  X-chromosome;  312 
Dobell,  58 

Doliolum,  budding,  7 
dominance,  in  embryo  hybrids,  305 
Drago,  167 
Drew,  106 

Driesch,  263,  264,  281,  301,  305 
Dromia,  cleavage  (superficial),  242 
dyads,  138 
Dzierzon,  315 

Echinoderm,  polarity  of  ovum,  92 
Echinus,    cleavage    under    pressure, 

284 ;   development   in   chemically 

altered     media,     320;  of 

isolated  blastomeres,  281 ;  number 

of  ova,  107 
ectoblast,  334 
ectoderm,  334 
ectoplasm,  34,  267 
ectosarc,  34 
Edwards,  311,  313 
eggs,  care  of,  104;  see  also  ovum. 
egg  membranes,  95,  ff.;  as  related  to 

conditions  of  development,  108 
egg  tubes,  of  Insects,  117,  118 
Elasmobranch,  blastoderm,  355 ; 

blastula,   344:   gastrulation,   343, 

ff;  344 

emboitement,  22 
embryo,  formation  from  germ  ring, 

347,  352 
Embryology,  defined,  2;  phases  in 

history  of,  20-21 
endoderm,  334 
endogamy,  189 
endoplasm,  34,  90,  267 
endosarc,  34 

end  piece,  of  spermatozoon,  100 
Entamoeba,     autogamy,     191,     192 ; 

maturation,  157 
enteroccelic  grooves,  336 
enteroccels,   in   Elasmobranchs  and 

Reptiles,  347;  in  frog,  342 
enteroderm,  348,  350 


enteron,  336 

entoblast,  334 

entrance  disc,  of  Nereis,  169 

Ephelota,  budding,  7 

epiblast,  334 

epiboly,  336 

epigenesis,  22,  23 

equational  division,  in  maturation, 

152 

equatorial  plate,  52 
Equisetum,  multipolar  spindle,  57 
Esox,  micropyle,  96 
Ethusa,  spermatozoon,  99 
Eudorina,  reproduction,  199 
Euglena,  nuclear  division,  61 
Euglypha,  nuclear  division,  60 
Eunice,  spawning  season,  103 
Euplotes,  fission,  6 
Euschistus,  dimorphic  spermatozoa, 

101 

evagination,  360 
exogamy,  189,  200 
exoplasm,     90;     of     ovum,     during 

fertilization,  174,  175 
external    conditions,    as    factors    in 

differentiation,  318,  ff. 

Farmer  and  Moore,  133 

fertilization,  13,  164,  ff.;  among 
Protozoa,  189,  ff.;  by  ''foreign" 
spermatozoon,  171,  172;  chemical 
processes  of,  205-206;  defined, 
165;  following  maturation,  187- 
188;  membrane,  175;  -  -  arti- 
ficial formation,  205;  mutual,  in 
Ciliates,  194;  of  enucleate  egg- 
fragments,  301-303;  preceding  or 
during  maturation,  180,  ff.;  re- 
lation to  heredity,  214,  ff.;  — 

rejuvenation,  209,  ff.;  212; 

-  reproduction,  17,  202, 
'.,  208,  ff.', variation, 


!13;  selective,  171;  significance  of, 
202,  ff.;  time  relation  to  matura- 
tion, 179,  ff.,  180 

filar  substance,  34 

fission,  in  Metazoa,  4-5;  multiple,  3; 
simple  or  binary,  2,  5 

flagellum,  of  spermatozoon,  98 

Flemming,  43,  44,  141 

Fol,  80 

follicle,  of  ovum,  96;  ovarian,  119- 
120,  119 

formative  stuffs,  278 

frog,  blastula,  340;  development  of 
single  blastomeres,  284—285;  en- 
terocoals,  342 

Frontonia,  nucleo-cytoplasmic  rela- 
tion during  interkinesis,  78 

Fundulus,  chromosomes,  hybrids  in 


INDEX 


371 


75,  223;  monsters  (cyclopean)  in 
presence  of  lithium,  318,  319; 
ovum,  94 

Gametes,  of  Metazoa,  13;  of  Pro- 
tozoa, 189,  ff. 

gametogenesis,  18 

gametogonidia,  10 

gametophvte,  chromosomes  of,  161- 
162 

Ganoids,  blastula,  331 

Garbowsky,  283 

gastrula,  19,  333,  ff. 

gastrulation,  333,  ff.;  in  meroblastic 
ova,  343,  ff. 

Gegenbaur,  20 

gemraules,  7 

genetic  continuity,  of  cell  organs, 
55 ;  of  chromosomes,  70,  ff. 

genital  ridge,  109 

germ,  14;  organization  of,  23,  25; 
predetermination  of,  264,  ff. 

germ  cells,  85,  ff.;  derivation  of,  112- 
113;  of  Protozoa,  189,  ff.;  relation 
to  breeding  habits,  102,  ff. 

germ  disc,  177,  332 

germinal  continuity,  theory  of,  14, 
15 

germinal  localization,  hypothesis, 
264,  ff. 

germinal  vesicle,  89 

germ  layers,  in  budding,  358;  in 
regeneration,  358;  morphogenetic 
value  of,  356,  358;  primary,  334; 
theory  of,  355,  ff.; ex- 
ceptions to,  357,  ff. 

germ  ring,  348;  of  Amphioxus,  334; 
of  frog,  340,  343 

"giant"  polar  bodies,  148,  149 

Godlewski,  171,  220,  221,  301,  304 

Gonactinia,  fission,  5 

gonads,  109;  epithelial  structure, 
112;  of  Metazoa,  13 

growth  period,  of  06-  and  spermato- 
cyte,  113 

guinea  pig,  metamorphosis  of  sper- 
matid,  126 

Gulick,  311 

Guyer,  311,  313 

Hacker,  135 

Haeckel,  280 

Hagedoorn,  301 

Haller,  22 

haploid    number    of    chromosomes, 

130,  133 

Hartmann  and  Xagler,  160 
Harvey,  202 

head,  of  spermatozoon,  98 
Heidenhain,  41,  80 
Helix,   chromosomal  variation,   67; 


"trophospongien"  in  hepatic  duct 
cells,  40 

Henking,  307,  310 

Herbst,  75,  271,  298,  303,  318 

heredity,  defined,  261;  mechanism 
of,  321-322;  relation  to  develop- 
ment, 260,  ff.; fertiliza- 
tion, 214,  ff.;  role  of  external 
factors  in,  317,  ff. 

hermaphroditism,  13 

Hertwig,  O.,  56,  114,  150,  213,  263, 
265,  281 

Hertwig,  O.  and  R.,  356 

Hertwig,  R.,  77,  157,  209,  210,  222 

Hesperotettix,   X-chromosome,    312 

heterochromosomes,  310 

Heterodontus,  ovum,  87 

heterotype  division,  140 

His,  264,  352 

histogenesis,  20,  363 

Holophrya,  multiple  fission,  3 

homolecithal  ova,  93,  226 

homotvpe  division,  141 

Hooke,  31 

human  ovum,  88 ;  spermatozoon, 
98,  100 

Huxley,  223 

hyaloplasm,  35 

hybridization,  evidence  from,  upon 
chromosome  hypothesis,  301,  ff. 

Hydatina,  relation  of  sex  to  fertiliza- 
tion, 315 

Hydra,  ovum,  88,  118 

Hydrophilus,  superficial  cleavage, 
243 

hypoblast,  334 

Ids,  idants,  292 
idiochromatin,  58,  158 
idiochromidia,  41,  156,  190 
idiochromosomes,     307;     variations 

in,  311,  312,  313 
idiosome,  125 
Inachus,  spermatozoon,  99 
indirect  cell  division,  44,  ff. 
Insects,    maturation,    140 ;    ovaries 
,    (egg  tubes),  117,  118 
interfilar  substance,  35 
interkinesis,  45 
intermediate  layer,  333 
internal  buds,  7 
interzonal  fibers,  53 
invagination,  336,  360 
involution,  336 
islands,  protoplasmic,  242 
isogamy,  192,  ff. 
isolated    blastomeres,    development 

of,  269,  ff.,  280,  ff. 
isolecithal  ova,  93,  226 
isotropic  blastomere  group,  263 


372 


INDEX 


Jenkinson,  318 
Jennings,  209    . 
Jordan,  106 
Julus,  attraction  cone,  168 

Karyogamy,  190,  192,  ff. 

karyokinesis,  44,  ff, 

karyolymph,  37 

karyoplasm,  37 

karyosomes,  38 

kern-plasma    relation,    77;    during 

cleavage,  222 
kinoplasm,  39 
Klossia,  syngamy,  197 
Korschelt  and  Heider,  152 
Kupelwieser,  172,  303 

Lacerta,  genital  ridge,  111 

Lankester,  264 

Lepas,  movement  of  spindle,  230 

Lepidoptera,  digametic  females,  314 

Lepidosiren,  chromosomes,  68;  mat- 
uration, 134,  136 

leptonema,  133 

Leptoplana,  blastomeres,  256 

Leptynia,  X-chromosome,  312 

Leudscus,  spermatozoon,  99 

Leuckart,  107 

Ley  dig,  31 

Lilium,  spireme  in  spore-mother- 
cell,  53 

Lillie,  F.  R.,  168,  183,  245,  275,  278, 
279 

Lillie,  R.  S.,  80 

Limax,  polar  bodies,  148 

Limnadia,  parthenogenesis,  203 

linin,  37 

lithium,  effects  upon  development  of 
Fundulus,  318,  319 

Litomastix,  multiple  embryo  for- 
mation, 315 

localization,  development  of,  286; 
germinal,  264,  288;  regulation  of, 
284,  286 

Locusta,  spermatophore,  105 

Loeb,  171,  172,  205,  206,  210,  301 

Loeb  and  Moore  (read,  Loeb,  King 
and  Moore),  305 

Loligo,  cleavage  (bilateral),  237,  241; 
spermatophores,  106 

Loricera,  spermatophore,  105 

Lott,  166 

Lyon,  269 

McClendon,  269 
McClung,  135,  310,  313 
Maas,  280 
macrogamete,  10 
macronucleus,  58 

magnesium,  effects  of  upon  differ- 
entiation, 318,  319 


malic  acid,  effects  of  upon  motion  of 
spermatozooids,  167 

Malpighi,  22 

Mammalia,  ova,  87 

mantle  fibers,  52,  79 

Mark,  148,  266 

Marshall,  103 

Mastigella,  chromosome  number,  66 

maternal  characters,  in  hybrids, 
301,  ff.;  affected  by  external  con- 
ditions, 304,  ff. 

maturation,  131,  ff.;  induced  ("arti- 
ficial"), 205;  in  oogenesis,  142, 
ff.;  in  parthenogenesis,  158,  ff.; 
in  plants,  place  in  life  history,  161 ; 
in  Protozoa,  156,  ff.;  in  spermato- 
genesis,  133,  ff.;  period,  of  op-  and 
spermatocyte,  113;  place  in  life 
history,  160,  ff.;  reducing  divisions 
in,  152;  relation  to  heredity,  152; 
results  of,  151,  ff.;  stimuli  leading 
to,  150;  time  relation  to  fertiliza- 
tion, 179,  ff.,  180 

Maupas,  209,  210 

megagamete,  197 

meiotic  (maiotic)  division,  133 

membrane,  chorionic,  96;  formation 
by  fertilized  ovum,  175;  nutritive, 
protective,  etc.,  97;  of  ova,  95,  ff.; 
tertiary,  97;  vitelline,  95 

Mendelian  heredity,  relation  of 
chromosome  structure,  294,  295 

Menidia.  chromosomes  in  hybrids, 
75,  223 

Mermiria,  X-chromosome,  312 

merocytes,  171 

merogony,  204 

mesoblast,  336 

mesoderm,  336;  axial  and  gastral, 
338,  339 

mesophase,  52 

mesoplasm,  267 

Mesostomum,  cleavage  (irregular) , 
239 

metamorphosis,  of  spermatid,  114, 
125,  ff.,  126 

metaphase,  52 

metaplasm,  40 

Metapodius,  idiochromosome,  311 

Metcalf,  62 

microgamete,  11,  198 

Micrometrus,  primitive  germ  cells  in 
embryo,  112 

micronucleus,  58 

micropylar  cell,  96,  120 

micropyle,  96 

microsomes,  35 

Microstomum,  fission,  5 

middle  piece,  of  spermatozoon,  98 

Minot,  210 


INDEX 


373 


mitochondria,  41,  125,  127 

mitome,  34 

mitosis,  44,  ff.;  causes  of,  76,  ff.; 
diagram,  48;  duration,  54;  in 
Protozoa,  57,  ff.;  in  Salamandra, 
46-47;  in  Unio,  50;  mechanism 
of,  79,  ff.;  modifications,  57,  ff.; 
of  cleavage,  219 

Moenkhaus,  75,  223 

monads,  138 

Monas,  anisogamy,  198 

monocentric  system,  79 

monodermic  organism,  330 

monoestrous,  103 

monogony,  12 

monosome,  310 

monospermy,  169 

Montgomery,  68,  101,  135,  311 

Moore,  135 

.Morgan,  269,  275,  276,  277,  301, 
311,  313 

Morgan  and  Spooner,  275 

morphogenetic  processes,  359,  ff. 

Morse,  205 

mouse,  amitosis  in  tendon  cells,  43 

multiple  embryo  formation,  315 

multiplication,  period  of,  during  06- 
and  spermatogenesis,  113,  114 

Mus;  see  mouse,  rat. 

Musca,  ovum,  91 

Myzostoma,  spermatozoon,  99 

Nageli,  265 

"Xebenkern,"  41 

neck,  of  spermatozoon,  98 

Nereis,  development  after  subjec- 
tion to  pressure.  281-282;  en- 
trance of  spermatozoon,  168,  169; 
fertilization,  174 ;  oocyte,  89 ; 
ovum,  95 

nests,  formation  of,  104 

neuroplasm,  267 

Xewman  and  Patterson,  315 

Nezara,  idiochromosomes,  313 

Noctiluca,  conjugation,  192;  nuclear 
division,  61 

notochord,  336 

notogenesis,  333 

Noturus,  blast ula,  220,  331 

nuclear  analvsis,  hypothesis  of,  265, 
288,  ff. 

nuclear  determination,  288 

nuclear  sap,  37 ;  as  a  factor  in  deter- 
mination, 298 

nuclear  substance,  synthesis  in  cleav- 
age, 220 

nuclein,  synthesis  in  fertilization, 
205-206 

nucleo-cytoplasmic  relation,  77,  78; 
in  senescence  and  rejuvenation,  210 


nucleolus,  38 
nucleus,  structure,  37-38 
nuptial  season,  103 
nurse  cells,  118,  119 
nutritive  membranes,  97 
nutritive  relations  of  growing  ova. 
118,  ff. 

Octets,  of  blastomeres,  229 

oestrus,  103 

oocytes,  primary  and  secondary, 
114,  115,  144 

oogenesis,  18,  113,  ff.',  diagram,  114 

oogonia,  114 

oogonidia,  10 

Opalina,  chromosomes  (amoeboid), 
54;  conjugation,  193;  nuclear 
division,  62 

Ophryotrocha,  nurse  cells,  119 

Orcheobius,  gametes,  198 

organ-forming  substances,  92,  267, 
275 

organization,  of  germ,  23,  25;  of 
nucleus,  288,  ff. ;  of  ovum,  91; 

during  fertilization, 

179; relation  to  cleav- 
age, 246 

Orthoptera,  idiochromosomes,  312 

ovary,  109 

Overt  on,  136 

oviparous,  104 

ovum  (or  ova),  amoeboid,  87,  88; 
animal  pole,  92;  comparison  with 
spermatozoon,  101,  150,  151;  cyto- 
plasmic  differentiation,  90,  ff.; 
demersal,  104;  deutoplasm  in,  93; 
follicle,  96,  119;  human,  88;  mem- 
branes, 95,  ff.;  nucleus,  89; 
numbers,  107,  ff.;  nutritive  rela- 
tions during  growth,  118,  ff.; 
organization,  91,  266,  ff.;  pelagic, 
104;  polarity,  91,  ff.;  position  with 
reference  to  gravity,  95;  promor- 
phology,  91;  reorganization  fol- 
lowing fertilization,  177;  sizes,  87; 
types,  87,  ff.;  vegetal  pole,  92 

Pachynema,  135 

palingenetic  traits,  27 

Palolo,  spawning  period,  103 

Paludina,  spermatozoon,  99 

Pander,  355 

Pandorina,  conjugation,  196;  repro- 
duction, 8-9,  9,  199,  200 

paralinin,  37 

Paramcecium,  chromosomes,  59,  64, 
296;  number,  66;  fertiliza- 
tion (conjugation),  194,  195; 
life  cycle,  210,  211;  maturation, 
157 


374 


INDEX 


paraplasm,  35,  40 

parasynapsis,  136 

parthenogenesis,  17,  203;  "artificial" 
204,  ff. 

parthenogonidia,  10 

Pasteur,  1 

Patella,  development  of  isolated 
blastomeres,  274 

paternal  resemblances  in  hybrids, 
affected  by  external  conditions, 
304,  ff. 

paths,  of  pronuclei,  in  fertilization, 
185,  186 

Patterson,  348 

Paulmier,  307,  310 

Payne,  311,  313 

Pelagia,  chromatin  extrusion  in 
oocyte,  299 

perforatorium,  of  spermatozoon,  98 

periblast,  346 

Peripatus,  sperm atophore,  105 

perivitelline  space,  176 

Petromyzon,  blastula,  220,  231; 
ovum  and  spermatozoon,  87 

Pfeffer,  167 

Pfluger,  263 

Phyllopneuste,  spermatozoon,  99 

pigeon,  gastrulation,  349 

plane,  of  cell  division,  55,  56 

Planocera,  cell  lineage,  248,  ff.,  252- 
253;  cleavage,  248,  ff.,  250,  251 

plasmosomes,  38 

plastids,  39 

plastogamy,  189,  190 

Plateau's  law,  231 

Platner,  114,  150 

Pleodorina,  reproduction,  9,  10 

polar  bodies,  115,  116,  144,  147; 
comparison  with  spermatids,  116, 
147,  148;  "giant,"  148,  149;  loca- 
tion, 149,  150;  size,  148,  149 

polarity,  of  cell,  41;  of  ovum,  91,  ff., 
266 

polar  lobe,  of  Dentalium,  271 

polar  nuclei,  149 

Polygordius,  cleavage  (radial),  234 

polycestrous,  103 

polyspermy,  170;  physiological,  170 

postgeneration,  284 

postreduction,  152 

potassium,  effects  upon  differentia- 
tion, 318 

predelineation,  23 

predetermination,  23 ;  of  germ,  264,  ff. 

preformation,  21 

prereduction,  152 

prespermatogonium,  121 

pressure,  effects  upon  cleavage  and 
differentiation,  281-282,  284 

primitive  groove,  348 


primitive  gut  cavity,  334 

primitive  streak,  346,  348;  of  Ser- 
ranus,  354 

primordial  germ  cells,  111 

Pristiurus,  chromosomes,  69 

promorphology,  of  germ,  264,  ff.; 
of  ovum,  91 

pronuclei,  behavior  during  fertiliza- 
tion, 180,  ff.,  186,  187-188 

prophase,  52 

prospective  potency,  263 

prospective  significance,  263 

protandry,  13 

Proteus,  chromatin  extrusion  in 
oocyte,  299 

prothallus,  chromosome  number, 
161 

protogony,  13 

protoplasm,  structure,  34-36,  36 

protoplasmic  bridges,  32,  42 

Protozoa,  gametes,  198;  mitosis,  57, 
ff.;  relation  between  fertilization 
(conjugation)  and  reproduction, 
208,  ff.;  reproduction,  2-11 ;  senes- 
cence and  rejuvenation,  209,  ff. 

Psammechinus,  development  of 
fused  embryos,  285 

pseudochromosomes,  41 

pseudocopulation,  105 

pseudohybHds,  303 

Pygosteus,  micropyle,  96 

Quartets,  of  blastomeres,  229 

Rabl,  41,  70 

Raja,  chromosome  groups  (auton- 
omy), 225 

Rana,  see  frog. 

rat,  spermatogenesis,  diagram,  122, 
123 ;  spermatozoon,  99 

recapitulation,  theory  of,  25-27,  26 

Redi,  1 

reducing  divisions,  133;  in  matura- 
tion, 152,  relation  to  chromioles, 
153 

reduction,  with  tetrad  formation,  in 
oogenesis  and  spermatogenesis, 
139,  146 

regulation,  of  localization,  284,  286 

rejuvenation,  relation  to  fertiliza- 
tion (conjugation),  209,  ff.,  212 

reproduction,  relation  to  fertiliza- 
tion (conjugation),  202,  ff. 

Reptiles,  gastrula,  345 

resorption,  in  morphogenesis,  362 

resting  period,  45 

reticulum,  protoplasmic,  34,  35 

Rhodeus,  ova,  104 

Rhoditis,  parthenogenesis,  203 

Rhumbler,  81,  189 


INDEX 


375 


Rhynchelmis,  cleavage  (radial),  236 
Roux,  91,  265,  271,  284,  329 
Ruckert,  75,  144 

Sachs-Hertwig,  law  of  cleavage,  229 

Sala,  282 

Salamandra,  mitosis,  46,  47;  sper- 
matogenesis,  124;  spermatozoon, 
100 

salts,  effects  upon  development, 
318,  f. 

Sauropsids,  gastrulation,  348,  ff. 

Schaudinn,  191 

schizogonv,  4 

Schleichef,  44 

Schleiden,  31 

Schonfeld,  133 

Schultze,  31 

Schwann,  20,  31 

Schweigger-Seidel,  20 

Scott,  103 

ScyUium,  egg  membranes,  96 

sea-urchin,  see  Arbacia,  Echinus, 
Sphcer  echinus,  Strong  ylocentrotus, 
Toxopneustes. 

secondary  sexual  characters,  in 
Protozoa,  200 

Seeliger,  301 

segmentation  (cleavage),  18 

segmentation  cavity,  221,  331 

seminal  fluid,  101 

seminal  vesicles,  129 

senescence,  210 

Serranus,  cleavage  (discoid),  240; 
primitive  streak,  354 

Sertoli  cells,  121 

sex,  determination  of,  306,  ff.,  315; 
heredity  of,  315;  limited  heredity, 
316 

sexual  and  asexual  reproduction,  12 

shells,  of  ova,  96 

Sida,  spermatozoon,  99 

Silvestri,  315 

Sinety,  313 

skein,  45 

soma,  14 

spawning  habits,  102,  ff. 

sperm  agonidia,  10 

spermatid,  115;  comparison  with 
polar  bodies,  116,  147,  148;  com- 
parison with  spermatozoon,  128; 
structure,  125 

spermatocytes,  133,  135,  ff.;  pri- 
mary and  secondary,  114,  115 

spermatogenesis,  18,  113,  jf.,  122,  ff.; 
diagram,  115 

spermatogonia,  114,  133 

spermatophores,  105,  106 

spermatosphere,  125 

spermatozoon  (or  spermatozoa),  97, 


ff.,  115;  accessory,  171;  agglutina- 
tion of,  167;  behavior  upon  enter- 
ing ovum,  172,  ff.,  181;  comparison 
with  ovum,  150,  151;  direction  of 
swimming,  166;  entrance  into 
ovum,  167,  ff.;  entrance  path, 
185,  186;  exclusion  of,  by  ova, 
170;  flagellate,  structure,  97,  98; 
forms,  97,  ff.  99;  "giant,"  101; 
head  of,  transformed  into  pronu- 
cleus,  181;  human,  size  of,  100;  lon- 
gevity, 166;  number,  100,  107-108; 
penetration  into  ovum,  167;  rate 
of  swimming,  166;  size,  100;  spiral 
path,  in  swimming,  166;  volume, 
101 

Sphcer echinus,  development  in  chem- 
ically altered  media,  320 ; of 

fused  blastulae,  286 ; iso- 
lated blastomeres,  281 ; 

parts  of  gastrulae,  283  ;  larva  from 
dispermic  ovum,  300;  pseudo- 
hybrids,  history  of  chrpmatin,  302 

spindle,  51;  chromatic,  in  Opalina, 
62;  intranuclear,  67;  multipolar, 
67 ;  origin  of ,  in  Protozoa,  62;  posi- 
tion of,  in  cell,  230 

spinning  activity,  149 

spireme,  45 

spongioplasm,  34;  arrangement  dur- 
ing mitosis,  79 

spontaneous  generation,  1 

spores,  development  from,  203;  for- 
mation, 4 

sporophyte,  chromosomes,  161—162 

squash-bug,  see  Anasa. 

star-fish,  see  Asterias,  Astropecten. 

statoblasts,  7 

Steinbruck,  305 

Stephanosph&ra,  conjugation,  193; 
reproduction,  199 

Stevens,  311 

Stockard,  318 

Strasburger,  80,  136,  162 

stroma,  of  ovary,  111 

Strongylocentrotus,  chromosomal  va- 
riation, 67;  cleavage,  233;  de- 
velopment of  isolated  blastomeres, 
282;  history  of  chromatin  in 
pseudohybrids,  304 ;  modified 
chromosomes  in  ova,  314 

structure  of  protoplasm,  34—36 

Styela,  see  Cynthia. 

sub-germinal  cavity,  332 

Sumner,  354 

Surface,  248,  249,  252 

symmetry,  of  ovum  and  adult, 
relation  to  first  cleavage,  246;  of 
ovum,  changes  during  fertiliza- 
tion, 179 


376 


INDEX 


synapsis,  135,  294 

Synapta,  cleavage  (regular),   227, 

228,  229 
syndesis,  135 
Syngamus,  107 
syngamy,  12,  13,  165 
synizesis,  135 

Tadorna,  spermatozoon,  99 

tail,  of  spermatozoon,  98 

Teleost,  gastrulation,  343,  346,  ff., 
347 

telolecithal  ova,  93,  226 

telophase,  54 

telosynapsis,  136 

Tennent,  304 

Tennent  and  Hogue,  207 

terminal  filament,  of  spermatozoon, 
100 

test  cells,  of  Tunicates,  120,  120 

testis,  109;  structure,  121 

tetrads,  136,  ff.,  139,  146 

Tetramitus,  conjugation,  193;  divi- 
sion, 59 

Torpedo,  blastoderm,  365 ;  poly- 
spermy,  170 

Toxopneustes,  entrance  of  sperma- 
tozoon into  ovum,  168;  fertiliza- 
tion, 173 ;  paths  of  pronuclei,  186 

Trachelomonas,  cell  division,  59,  64 

Trichomastix,  autogamy,  191 

Trichosomum,  107 

tridermic  embryo,  333 

Triton,  blastula,  331;  gastrulation, 
341 

trophochromatin,  58,  158 

"trophospongien,"  40,  41 

Tyndall,  1 

Unequal  division,  in  cleavage,  231 
Unio,   blastomeres,  256;  cleavage 

(radial),  235;  mitosis,  50 
Urodeles,  spermatogenesis,  124 
Urospora,  gametes,  198 

Vacuoles,  39 

variation,  relation  to  fertilization, 
213;  relative  amount  in  gameti- 


cally  and  agametically  produced 
individuals,  214 

vasa  deferentia,  129 

vasa  efferentia,  129 

vegetal  pole,  92 

Vernon,  304 

Vesperugo,  spermatozoon,  99 

Virchow,  20,  42 

vitelline  membrane,  95 

viviparous,  104 

Volvox,  gametes,  198 ;  reproduction, 
9-11,  11,  199,  200 

Vorticella,  fertilization  (conjuga- 
tion), 196 

DeVries,  265 

Wallace,  313 

Weismann,  14,  203,  213,  265,  292 

Whitman,  162,  264,  352 

Whitney,  269 

Wilson,  76,  101,  136,  168,  185,  231, 

247,  257,  263,  269,  271,  280,  281, 

287,  298,  311,  313,  314 
Winiwarter,  133,  135 
Wolff,  22,  355 
Woodruff,  211 

X-chromosomes,  307;  in  maturation, 
diagram,  309;  in  relation  to  sex, 
diagram,  310;  variations  in,  311, 
312 

Yatsu,  183,  207,  287 

Y-chromosomes,  311,  312 

yolk,  arrangement  in  ova,  93;  lobe, 
of  Dentalium,  271;  nucleus,  89; 
pyramids,  242 ;  relation  to  develop- 
mental period,  108;  sac,  350; 
stalk,  351 

Zeleny,  287 

Ziegler,  81 

Zoja,  280 

zona  radiata,  95 

zoospores,  4 

Zur  Strassen,  253,  255,  282 

zygonema,  135 

zygote,  11 


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